I think the best "nurture" case for homosexuality is probably situational homosexuality (prison and other gender-segregated total institutions) or cultural construction of sexual practices (IE MSM versus self-identified LBG individuals; Roman times where self-identified heterosexual citizens would practice homosexual acts with those of the slave/chattel as a sort of property relations).
But it's kind of a moot point because if you're educated enough on sexual politics to be discussing cases like that, you're also sensible enough to recognize that the "choice" question is not about determining truthfully if orientation is a choice, but rather about marginalizing it as a mutable, changable, treatable, fixable aberration--which is not an acceptable approach to sexual orientation regardless of its nominal original.
Your gay kids can have kids--they can do it genetically in a variety of ways, but more importantly, they can also adopt. The grandkids your gay kids have will be just as much yours whether or not you have a biological connection to them. Moreover it's quite possible that your straight kids will choose not to have kids, will be biologically unable to, or will choose to adopt. I'd seriously rethink rationalizing a prejudicial approach to your kids based on some tangential connection to a biological imperative.
It's clear you're not trying to be bigoted--but when you say, in any way, that being gay is a worse outcome for your kids than being straight, what that's say to gay people is that their identity is a worse outcome than if they had been straight. It is hurtful.