Anal intercourse is not an exclusively gay activity and is popular among hetero couples as well. With a minimum of precaution, ie. using lube, condoms, and practicing good hygene, it doesn't have to lead to health problems. Also, the anal sphincter muscles are just like any muscle in the body and can be stretched, or overstretched, but this also doesn't have to lead to problems.
I'll agree that people who are promiscuous or engage in extreme sexual activities such as fisting, using "toys", and drugs, regardless of the gender makeup of the partners, are taking equally extreme risks with their health, and it can be argued that they have a mental disorder which drives them to behave this way. Within both communities, gay or straight, there are people who engage in extreme behavior, and I believe that both communities are parallel in terms of their sexual proclivities. Given the statistics involving married people who cheat on their spouses (70% of men and close to 50% of women), I would say that promiscuity is alive and well in the straight community. Certainly among various groups within the straight community, say college students, professional athletes, and politicians, promiscuity is rampant. Visiting your local porn shop you'll find more toys for straight people, and more straight people buying them. Further, and as I'm in this business I can say this for a fact, there are at least 300 new porn titles available on a pay-per-view basis on your local cable TV, satellite provider, or in your Marriott hotel room, every month, and not one of them is gay. None. I am not trying to justify this behavior, but clearly there is no basis to any claim that the gay community has a greater tendancy toward extreme sexual behavior, unless you include anal sex among monogamous couples. That is hardly extreme compared to other behaviors, and gay couples who are in committed monogamous relationships have no more health issues than straight ones.
Further, the claim that the average life expectancy of gay men is X years is not valid. This number is primarily based on studying obituaries. Certainly AIDS has taken its toll, but there is no way of knowing the average age at death of those gay people who weren't active in the gay community or public about their sexual preference. Using the same statistical methods, one could argue that the average life expectancy of a bodybuilder using steroids is very young as well.