As you hopefully have read, I often point out the Importance of Sex for Long Term Male Health and even show ways that guys can Get More Sex without Being Manipulative. However, I certainly don't want to give the impression of being overly "casual" in this regard and I desire for everyone to be safe and to "return home for Christmas" to their friends and families. The great rewards of sex can carry with them certain risks as well, chief among these being the risk of STD's.
Few guys understand that an infected woman can literally shorten their lifespan and have other tragic long term consequences even if they take every antibiotic on planet earth. It always pays to know the truth (and, above all, your partner). First, let's look at the really dangerous stuff out there:
NOTE: See my link on Herpes and Arginine for how some of the new erectile supplements may fuel herpes issues in sensitive individuals.
1) HPV (Human Papillomavirus). Most guys that know anything about HPV at all believe that it is something that just affects women and can cause them cervical cancer. Some guys may have even heard that there are new vaccines out there called Gardasil and Cervarix that can protect women against many of the common strains of HPV, including HPV-16 and HPV-18.
However, the reality is that guys are from immune from the effects of HPV. For example, HPV can significantly increase the risk of oral or mouth cancer in males.  One study found that having at least six oral sex or open-mouthed kissing partners were correlated with an increased risk of getting HPV. That makes sense of course - the more partners you have, the more oral contact you have, the more your chances for HPV.
Oral cancer risks are also increased with smoking and drinking. If you have been even a moderate drinker and smoked part of your life, you are at even increased risk oral cancer and throwing an HPV infection on top of that will only increase your risk further.
HPV can also increase a guy's risk of anal and penile cancers  as well. Wikipedia is also reporting that HPV may contribute to some head and neck cancers. This makes HPV is one of the most serious of all sexual health issues for males (and females).
NEWS FLASH: HPV has been linked in mulitiple studies to skin cancer, specifically squamous cell carcinoma (and not basal cell).  A recent study found that HPV infection increased risk by about one-and-a-half times. 
Even worse for us guys is the fact that there is no FDA-approved way for males to even be tested for it. In other words, you can walk around with HPV and never even know you have it. In the meantime, about all you can do is Boost Your Immunity or, of course, know your sexual partner and her histroy extremely well.
NOTE: Penile cancer is fairly rare, accounting for about 1% of male cancers in the U.S. .
2) HIV or AIDS. HIV is much more under control. I remember going into an AIDs ward in New York City around 1990: it was the most miserable place I have ever been. Men laying in bed completely immobile with glazed eyes; a woman with a tongue swollen to twice its normal size and so on. Now they have the "triple cocktails" and a host of other drugs to keep the disease under control. No one knows for sure but most experts believe that lifespan will be shortened for those who HIV-infected. Also, keep in mind that some people have significant reactions and toxicities to the current AIDs medication, leaving them with an average remaining lifespan of around ten years.
3) Chlamydia. This bacteria can make you and your partner infertile. One recent study found that it imparied fertility in both males and females.  In females, it can lead to Pelvic Inflammatory Disease and permanent damage to the fallopain tubes, making pregnancy difficult or "impossible". NOTE: Every sexually active female should get tested once a year for this bacteria. In addition, see the SOLUTION below under HSV.
4) HSV-2. There is some evidence that this herpes simplex virus, more commonly called "genital herpes", can also increase one's risk of oral cancer when HPV is present as well. 
5) HSV-1. This is the common "cold sore" virus and not really a true STD since it can be contacted in many ways besides just sexual contact. It is also nearly ubiquitous and estimates are that 60% of the population is infected with it. However, you can definitely become infected through sexual contact and one should be aware that HSV-1 has been implicated as a risk factor for heart disease. In fact, one study of older subjects found that previous HSV-1 infection was associated with double the risk of death from heart attack and/or heart disease.  SOLUTIONS? One study found that Mercury and other heavy metals were a root cause of recurring herpes and chlamydia infections and used Chinese parsley as a treatment protocol.  The reason? Chinese parsley binds to heavy metals so that they can be eliminated from the body. Other plants with evidence of similar properties are cilantro and regular parsely. Yet another study found that tansy helped with herpes. 
So be careful! In another link, I show the Odds of Catching an STD, assuming you do not test and "spin the wheel or the revolver" and hope for the best.
Are there any solutions? Of course, it's well known that regular condom use can provide substantial protection against chlamydia and HIV (also gonorrhea and trichomoniasis). What was found in one 2009 study was that it even provided a 30% protection rate against HSV-2, which can be spread just by skin contact in certain cases. 
In addition, it looks like Gardasil has already been approved for males aged 9 to 26, which will provide protection from some of the major strains (6, 11, 16 and 18) that cause most of the various cancers mentioned above. It is interesting, because the FDA actually approved this for usage to prevent genital warts, but it will undoubtedly have a protective effect for cancer as well. NOTE: I am not completely sure why they capped the age at 26 and yet covered pre-puberty aged boys. Females have the choice of Cervarix as well.
2) Human Reproduction, 19(5):1121-1126."Demonstration of Chlamydia trachomatis IgG antibodies in the male partner of the infertile couple is correlated with a reduced likelihood of achieving pregnancy"
3) Amer J of Epidemiology, 1992, 135(10):1093-1102
4) Circulation, 2000;102:2335, "Chlamydia pneumoniae, Herpes Simplex Virus Type 1, and Cytomegalovirus and Incident Myocardial Infarction and Coronary Heart Disease Death in Older Adults"
5) Arch Intern Med. 2009, 169(13):1233-1240, "A Pooled Analysis of the Effect of Condoms in Preventing HSV-2 Acquisition"
7) Journal of the National Cancer Institute, 2006, 98(6):389-395, "Human Papillomavirus Infection and Incidence of Squamous Cell and Basal Cell Carcinomas of the Skin"
8) BMJ-British Medical Journal (2010, July 9). HPV infection linked to increased risk of skin cancer. ScienceDaily. Retrieved July 10, 2010, from http://www.sciencedaily.com /releases/2010/07/100708193448.htm9) Acupunct Electrother Res, 1995 Aug-Dec, 20(3-4):195-229, "Role of mercury (Hg) in resistant infections & effective treatment of Chlamydia trachomatis and Herpes family viral infections (and potential treatment for cancer) by removing localized Hg deposits with Chinese parsley and delivering effective antibiotics using various drug uptake enhancement methods"