I suspect that producing offspring is, from an individual's point of view, generally an unintended side-effect of (some) sexual activities. Yeah, it has an evolutionary/social purpose but so are a lot of impulses that we don't necessarily have to or try to understand in terms of cause and effect. But I may be wrong. I often am. Just ask my wife.

But I don't think a lot of homosexuals come away from a sexual encounter thinking "Damn, I'm not pregnant again!"