You're confusing 'lifelong relationship' and just plain being gay. Just like with straight people, gay people don't necessarily have to have a lifelong relationship.
The discussion is, as far as I know, about whether people are born gay or not, with 'gay' being defined as being attracted to people of the same sex. This has nothing to do with whether or not they have relationships. Therefore, it is completely possible (and in fact true, from personal experience) that people are born gay just as people are born straight.
Aside from that, I also fail to understand what 'role' is constructed. There is no role. I like men, I'm attracted to men, men make get me hot and bothered. So do women. I've been like this since I started thinking about sex in an adult fashion (beyond the pre-pubescent ideas of sex, where I really didn't think about men or women in particular), and so I presume that I've been like this since I was born and that a switch wasn't just suddenly thrown when I hit puberty or a certain age, and I know I didn't 'decide' to be this way one day while I was looking at a dirty magazine.
Outside of a 'civilized' society, you're probably not going to be able to determine anything about homosexuals for many reasons. For all you know, they're killed as soon as they're identified and so they hide completely. Or they simply supress it totally because that's the way the society is. Or they simply DO NOT TALK ABOUT IT even if it's going on. Or they happen rarely in a particular society due to the small size and the admittedly low percentage in our own society ('uncivilized' societies tend to be small because they lack all the things that we have which allow them to grow). And again, we're not talking about whether they have relationships, we're talking only about the fact of whether or not they have homosexual feelings. So simply saying that we don't hear about homosexual relationships outside of our 'civilized' society does NOT prove that there is no such thing as a homosexual in 'uncivilized' societies (or even that other 'civilized' societies don't have them, since they could also radically supress them).