Gay people don't know from a young age. I went out with a gay guy for over a year (knowing he was gay, christ do I have issues! Ha!) and he did not know he was gay until he was 24. (repression is a powerful thing).
I also don't think gay people destroy their lives, nor can they "choose". I know many very happy gay men. Including gay couples who've been together for 20 years and gay people can marry and adopt children if they want a family. Deadly diseases affect hetrosexual people too and, in fact, hetrosexual women is the one of the demographics with the highest growing HIV rate.
I'm not having a go. I'm sure you meant well. But the way you worded your reply almost sounded like it was somehow unfortunate to be gay, or that it should be avoided. If this guy enjoys his sexual encounters with men he should continue to enjoy them.