My suspicion is more that straight people are the ones who have children and they want their children to be like them. The relationships question is downstream of that - it would be silly if your children do turn out to be homosexual to want them to enter a hetero relationship; that just seems like a recipe for more trouble.

I say this as someone who has never really "gotten" the whole having children thing, so just my take as someone who may have a bit of an outsider perspective.