Yes, Sex is Good for You as a male in a dozen different ways. But sex can also kill you in a hundred different ways if you're not careful. The reason is that there are a number of potent STD's that could kill you, or at least "kill you earlier" by shortening your life span. The answer to that question I am going to cover below and it is admittedly rather sobering:
We will look at HIV, but let's start with the much more common HPV, Chlamydia, and HSV-2. As I covered in another link on STD's, HPV can kill males through both oral or mouth cancer and/or penile cancer and HIV through many pathways. Chlamydia can lead infertility and HSV-2 to genital warts and feeling sick during outbreaks.
So what are your odds if you just grab someone off the street and engage in some kind of risky behavior. Consider that 15-19 year olds have these odds of having the following STDs :
1) HPV: 24.5%
2) HSV-2: 2.3%
3) Chlamydia: 4.6%
In other words, a typical, average, normal 15-19 year old has about one in four chance of having HPV. However, most guys reading this site aren't dating underage teens. Consider this: the HPV rate, according to the same study , skyrockets to 44.8% for an average 20-24 year old, putting that age bracket to almost 50/50. And that makes sense: with each year or two that pass by, many women have additional partners and often accumulate different STDs, especially the viruses that have no cure. In other words, imagine the numbers for a woman in her 30's or 40's that has had many partners. Your odds of being exposed to HPV are much higher.
NEWS FLASH: Chlamydia was just found to cause DNA mutations that would likely cause cancer.  Specifically, researchers found increased "breaks" in DNA strands.
It's not wonder, because those same surveyors found that a high percentage (of young women at least) use little to no protection. Looking at the numbers you can see that about 40% of 20-24 year olds left themselves vulnerable to an STD.  Now perhaps older couples are more careful, but "humans are humans" and I doubt the numbers are that much different for any age bracket.
1) 20% have had intercourse with "no method"
2) 11.8% used "the pill".
3) 8.1% used the withdrawal method
This is further supported by the 20-24 year old stats on race, where the first, second and third numbers are percentages by white women, black and hispanic, respectively. 
1) No method 10.3 25.2 40.8
2) Pill 40.7 27.8 20.0
3) Withdrawal 14.3 ** 11.
Notice that in all cases the numbers add to between 50 and 75%.
In other words, if you change partners, it is more than likely that she had unprotected sex in the last year, at least for 20-24 year olds. Again, I doubt that any age range is significantly different.
Calculating your odds of HIV infection is much more difficult as I could find no numbers by age range and gender for infection rates. But it is possible to come up with a rough idea. We know that as of 2006 there are about 1.1 million infected individuals in the U.S. of all ages.  Of this, about half (48.1%) are "men having sex with men".
This means there are about 570,000 heterosexuals with AIDs and if we assume about half of those are females, that is 285,000 women. Another reasonable assumption is that in most cases the females will be between 20 and about 44 years old. Looking at standard demographic charts (for 2008), we can see that there are about 51.4 million females between these ages which puts your odds at roughly 1.1% or 1 in 89.
CAUTION: While it is true that it is less likely to catch HIV from oral sex, the odds are not miniscule. One study of HIV positive men for example estimated that about 6.6% of cases were from oral transmission.  Putting that in other terms, one study found that there was between a .04 and .06 percent chance of HIV infection with each oral sexual contact.  These are small numbers, but repeated oral sex with an infect partner will likely put you in the single digit risk percentages.
So how can you protect yourself? How can you preserve your health and life from these deadly diseases? Well, there are not a lot of good options other than testing, which is of course quite awkward. A condom is not going to protect you from HPV, for example.
A significant block of women are getting vaccinated (Gardasil and Cervarix) for some strains of HPV, but for a huge percentage women it is too late. There is some chance that science in the next decade or two will conquer some of these foes as well. The medical advances are quite astonishing. But, for now, "forewarned is forearmed" is about all that can be said.
1) http://www.cdc.gov/mmwr/preview/mmwrhtml/ss5806a1.htm#tab2, TABLE 4. Selected measures of pregnancies, births, birth characteristics, induced abortions, cases of human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS), sexually transmitted diseases (STDs), and sexual violence among females aged 10--24 years,* by age group --- National Vital Statistics System and multiple surveillance studies, United States, 2004--2006.
2) http://www.cdc.gov/mmwr/preview/mmwrhtml/ss5806a1.htm#tab2, TABLE 3. Proportion of sexual behaviors, exposure to prevention activities, and pregnancy wantedness among males aged 15--24 years,* by age group --- National Survey of Family Growth, United States, 2002
3) http://www.cdc.gov/mmwr/preview/mmwrhtml/ss5806a1.htm#tab2, TABLE 6. Proportion of sexual behaviors, exposure to prevention activities, and pregnancy wantedness among females aged 15--19 years,* by race/ethnicity --- National Survey of Family Growth, United States, 2002
4) MMWR, "HIV Prevalence Estimates -- United States, 2006", http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5739a2.htm
5) 7th Conf Retrovir Oppor Infect Jan 30 Feb 2 2000 Conf Retrovir Oppor Infect 7th 2000 San Franc Calif. 2000 Jan 30-Feb 2; 7: 165 (abstract no. 473), "Primary HIV infections associated with oral transmission"
6) Am J Epidemiol, 1999 Aug 1;150(3):306-11, "Per-contact risk of human immunodeficiency virus transmission between male sexual partners"