A better question is why heterosexuals are so unhappy.
Since you didn't bother to cite any sources or state actual statistics, let me do your homework for you.
There was a Canadian study back in 1997 that showed that homosexual men in Vancouver B.C. had an 8-20 year shorter lifespan. This was right when HIV deaths were peaking. This study is still cited by anti-gay web sites as proof that "fags die young" and is continuously repeated even though subsequent studies have shown this trend is reversing.
Unfortunately male homosexuals still account for 61% of new HIV infections in the U.S. This is certainly the behavior that is keeping the average lifespan of male homosexuals so low, not smoking and drinking.
I've found no study on "cancer reates [sic]" in homosexuals and I doubt you've found any either.
I've also found no study on the lifespans of lesbians, probably because it's not considered a national health issue. There was a Danish study in same-sex marriages (both male and female) which showed increased mortality but male homosexuals were five times more likely to die than lesbians which left the lesbians at about average. This study also had a sampling flaw:
So that study wasn't very convincing.
What crazy study are you pulling this ridiculous information from? Who are these drinking and smoking homosexuals who you've never met? The lesbians I know play sports and run marathons with me. The gay men I know go to the gym three times a week. They make us straight people look like lazy bums.
Since you've not cited any evidence that is true, it is NOT open for debate.
My experience with heterosexual couples is that they get married and have kids, then they stop taking care of themselves, they get fat and have health problems. I don't have statistics to back this up but you didn't have any either.