I see young men on the Peak Testosterone Forum with low testosterone all the time. In fact, probably around half of the regular posters are young men under 40. And usually these younger guys with low T are miserable - even more miserable than the older men. They also are panicked, because this threatens their sex life. This means that they are often very anxious to solve their problems with HRT (testosterone therapy). And in most cases, this works well as far as symptom relief and does indeed make them feel better. (In my opinion, young guys should do everything possible NOT to go on traditional TRT. Many other options are available that should be attempted first.)
Unfortunately, many doctors forget to tell their patients just one little detail: HRT dramatically lowers sperm counts! For example, one of our posters asked forum member the following story:
"I went to my GP about 9 months ago with the usual symptoms and tested at total T of 360. (I'm 35.) He gave me a 400mg shot "to try" and I seemed to respond. Retested about 3 months later at 280, got another huge 400mg shot and seemed to respond again. So he went ahead and prescribed me test enanthate, and I've been shooting about 50mg every 5 days* with so-so results." 
Then he asked this question: "I still want kids. Have I sterilized myself? My doc said that fertility is likely to be off already with low T and that replacing to physiologic levels won't suppress it further -- and may help. If I understand what I've read here, this is NOT the consensus view."
So this doctor not only did not tell him about the issue, but actually said that HRT might improve his fertility! Apparently, this doctor did not realize that testosterone is actually a pretty significant contraceptive. (Don't rely on it though!) For example, one study of Chinese men looked at testosterone undecanoate, which is now approved as Nebido here in the U.S., and found that it was incredibly effective as a contraceptive: "the mean serum testosterone concentration increased 131%, and the mean serum LH and FSH concentrations decreased 72% and 70%, respectively, after TU injections during the treatment period." 
Of course, the very significant decrease in FSH will lower fertility substantially and this was verified by the fact that 97% of the men developed azoospermia or severe oligozoospermia, i.e. a sperm count less than 3 million per ml, which would make pregnancy very difficult. And this is certainly not the only ester that causes infertility. Testosterone enanthate has been noted by researchers to cause infertility for example.  And enanthate is very similar to the cypionate ester that is so often used here in the U.S. as part of an HRT regimen.
And sure there are men who successfully get their woman pregnant while on HRT. It only takes one little swimmer to do it, eh? However, it is more the exception than the rule. Any loss of fertility can be a huge stressor for some couples. Women in particular can become despondent and depressed, but the stress can really hit the male as well. One study on infertile couples showed that ACTH levels rose in the guys, indicating HPA difficulties. And this increased stress caused lower sperm counts and motility in the study participants.  Thus, infertility can create even greater infertility and create a downward spiral.
CAUTION: Because of all this, you want to do all you can to stack the cards in your favor if you want kids. Below I give you some of the standard treatment methodologies to handle low testosterone and preserve or increase fertility, so that you can discuss things with your doctor and ask all the right questions. Don't forget that there are both "production issues" and "obstruction issues." Male factor infertility is not always just a hormonal issue and sometime you can get "blockage" issues. It's always smart to talk to a fertility specialist if you can afford it. Some urologists even handle fertility and have built up this expertise. Take some of the discussion points on this page to him or her.
So, if HRT causes such a loss in fertility, then how can a man boost his T and preserve his fertility at the same time? Or does he have to choose between one or the other?
It turns out that there are a number of options for men in this category that can solve these sorts of issues. Let's take a minute and look at each:
1. HCG Monotherapy (with HMG as needed). One option that I see is an increasing number of men undergoing HCG Monotherapy, as opposed to the standard "HRT Trifecta" of testosterone cypionate, HCG and Arimidex. HCG is commonly given now to men in conjunction with testosterone to preserve testicular size/volume primarily. However, HCG can also be given by itself in order to stimulate the man's own natural testosterone production. The reason that this works is that most men with low T actually have secondary hypogonadism where the pituitary or hypothalamus is the broken link. As it turns out, HCG is an LH (leutinizing hormone) analog and will mimic the effects of LH on the testes, i.e. stimulate them to produce testosterone, assuming the testes are in good shape of course.
I should also mention that usually some low dose Arimidex needs to be taken with this kind of treatment, because estradiol levels can ramp up significantly. So it is monotherapy in the sense that no testosterone is given, but, in reality, there is often a second pharmaceutical involved. For more information, see my link The Hows and Whys of HCG Monotherapy.
And, actually, sometimes HMG is added to the regimen as well if a man's fertility parameters (from a semen analyis) have not responded after a few months. HMG is actually a combination of FSH (follical stimulating hormone) and LH, with each brand being a little different formulation. The FSH actually much more directly stimulates sperm production than HCG, which primarily induces testosterone production.
The HCG/HMG combination can take awhile to work, usually around 6-9 months before you begin to see reasonable increases in sperm counts and so on. This method is not foolproof and has a mixed track record.  Your fertility doctor can give you a good idea as to whether or not you are a good candidate.
2. HCG + HRT. Dr. Lipshultz has several studies under his belt showing that most men on HRT can maintain their fertility if they add in low dose HCG. One study put men either on injections or daily transdermal (topical) testosterone gels and then added in 500 IU every other day of HCG. The abstract states the remarkable results that "no impact on semen parameters was observed as a function of testosterone formulation. No patient became azoospermic during concomitant testosterone replacement and human chorionic gonadotropin therapy. Nine of 26 men contributed to pregnancy with the partner during followup."  Fertility is so important that I encourage you to talk to fertility specialist and come up with a treatment plan that will work for your situation rather than just go by this one study. But, yes, it looks like you can have your cake and eat it too!
3) Clomid. When you think of fertility medications, Clomid is probably the name that first pops into most everyone's mind. It is used by both men and women, of course, and is often the first line medication because of it's convenience, relatively low cost and ability to stimulate sperm production. What could be simpler? No injections. No needles. Just take a pill.
And it works very well for some men, who get both nice boosts in testosterone, sperm counts and libido. We had a man on Peak Testosterone Forum whose testosterone went up to 1500 ng/dl on Clomid! Sot it can give a substantial boost. One study found more modest, yet significant increases from 248 to 610 ng/dl with a 25 mg daily dosage. (See my link on Clomid and Testosterone for details.) And sperm counts can go from a couple million to a 100 million in just a couple of months as well.
However, the reality regarding Clomid is far from the ideal generally speaking and there are a number of persistent issues that plague this medication:
a) There is very often no libido boost whatsoever in men taking Clomid. Their testosterone may triple and yet they have no real increase in sexual desire.
b) It often stops working or at least loses efficacy after a few months. Some women interested in getting pregnant bypass this concern by trying to "harvest" their husband's sperm during those few months for a future IVF. Yeah, that probably won't be the last time you're milked...
c) Clomid can display estrogen-like effects in the brain and increase moodiness and other related symptoms.
d) There are some concerns about long term effects and I outline these in my link on Potential Long Terms Risks of Clomid Therapy.
Again, some men seem to thrive on it and do very well and, unfortunately, there is no good way to predict who will do well and who will not.
4. Armidex Monotherapy. This is not very common from what I have seen, but some fertility specialists will put young men on Armidex by itself. Men get their estradiol, the most potent estrogen, through an enzyme called aromatase (that is stored primarily in fat cells) that converts some testosterone into estradiol. Arimidex (anastrozole) is an aromatase inhibitor and thus slows down this conversion of testosterone into estradiol. Therefore, it will raise both testosterone and lower estradiol simultaneously and can preserve fertility. (It doesn't seem to have a good reputation for actually raising fertility, but talk to your doctor about this.)
How much can Armidex raise testosterone? You can read about it more in my link on Testosterone and Arimidex, where I discuss a study where Arimidex increased testosterone by 62% and decreased estradiol by 24%.
5. Combinations of the Above. You can find every combination imagineable. HCG + Arimidex; Clomid + Arimidex; HCG + Clomid + Arimidex. And some doctors use letrazole instead of Arimidex (anastrozole). If you go to a fertility specialist, he will test you in every way - ultrasounds, hormone blood draws, genetic and semen analysis, etc. - and then will decide on the best course of action.
1) The Journal of Clinical Endocrinology & Metabolism, Feb 1 2003, 8(2):562-568, "A Multicenter Contraceptive Efficacy Study of Injectable Testosterone Undecanoate in Healthy Chinese Men"
2) The Journal of Clinical Endocrinology & Metabolism, Oct 1 1993, 77(4):1028-1032, "Body composition and muscle strength in healthy men receiving testosterone enanthate for contraception"
4) Neuroendocrinology Letters, 2005, "Levels of plasma ACTH in men from infertile couples"
5) Andrologia, 1981 May-Jun, 13(3):187-97, "A modern approach to the gonadotropin treatment in oligozoospermia"
6) The Journal of Urology, February 2013, 189(2):647ï¿½650, "Concomitant Intramuscular Human Chorionic Gonadotropin Preserves Spermatogenesis in Men Undergoing Testosterone Replacement Therapy"