Until someone can actually articulate how gay marriage affects children / nuclear family, I'm going to write them off as concern trolling.
If they simply wave around the whole "but what about the children," without actually understanding WHAT about the children, I see that as concern trolling (just pretending to care about x, while really just saying y.)
Personally, I don't think the anti-gay marriage crowd actually knows anything about the effect of gay marriage on children. I'm pretty sure it's just a talking point for them-- sure, they use it amongst themselves, but only to give each other ammo to concern troll the "opposition."
That's why I see it as concern trolling... none of them actually know about any "real world effects" on children (because... there are none). They use it as a talking point.
They don't need to be able to articulate how gay marriage affects children or the nuclear family in order to be sincere in those concerns, though. Whether their arguments are plausible is unrelated to the question of whether their arguments are sincerely held, and on that argument, at least, I think they are. That's the litmus test for whether something is concern trolling; whether it is something you are actually concerned about, or whether it is something that you are expressing faux-concern over in order to undermine a position you dislike.
You're right that it's a talking point with no basis on reality. That doesn't make it "concern trolling"; it just means that it's baseless and irrational.