I find it scary when I'm introduced to the sort of people who feel that two role models, no more and no less, are necessary for a child's development. Gods forbid that someone not related by blood or marriage take a strong role in their precious child's life. Gods forbid that someone not hand-picked by them take a role in their precious child's life. Gods forbid that someone who doesn't think like them should take a role in their precious child's life. And gods forbid that maybe the kids would like one primary caregiver who isn't stressed to hell and back rather than two primary caregivers who dislike, disrespect, resent, rant, and rave at each other.
Children need someone to be there, love them and hug them, pay attention to them and what they're getting into, tell them "No!" when they want something that will be bad for them, and teach them how everything's supposed to work, mostly by example. More than a commitment to a partner, a parent has to make a commitment to the kid, to be there in any possible way. It's far more damaging to a child's future mindset if parents treat one sex like second-class citizens than if all parents are one sex. (And yeah, that still goes for the women who do the "All men are scum! Scum, I say!" thing, as well as the "Women are wicked and beneath me and deserve to be treated poorly because of it" thing.)
Back in 2001, I wound up roommates with a single mother and her son, and I'm fairly well an aunt by now, and the kid is 7 and can't remember a time before me clearly at all. The one thing that's made him the most unhappy over the years has been when people who were close to the household (roommates, neighbors, friends) stopped spending as much regular time with the household.
His criteria are not "Are they straight? Gay? Bi? Pagan? Christian? Jewish? Married? Partnered? Single? Poly? Swinger? Mono?" He's interested in if they'll play with him, and if the games they want to play are fun.