I'm not a lesbian parent
.
I live in a super-liberal American city, where there's enough general knowledge and awareness for this to just not happen.
Thinking of the lesbian moms I know, I think their response to such a comment might be (1) to ignore it/pretend not to have heard it because they're over everything relating to their being lesbian parents (2) answer openly that it does matter, but without negativity (ie just explaining that it does matter, and maybe why if they know the person well enough) (3) eye-roll and say "actually, it does" and move on (4) thinking of one couple where one woman went through traumatic cancer treatment which left her unable to carry a baby to term, she'd probably explain the truth, which is that she would desperately like to have been pregnant and that she wishes they could have both had the experience.
I think, in all probability, they would probably do (1), just to close the door on the inevitable follow-up of "so, whose sperm did you use?".
Some people just don't know. Some don't want to know, some are nosy, some are respectfully interested but don't know if/how to ask. Whatever, it's not your problem or your responsibility to educate if you don't want to. Round these parts, people just get on with life. And, my kids are older now and parents don't figure so much. If you're still in the thick of baby classes etc, maybe come up with a reply that you're comfortable with and use it at all times. There's nothing wrong with "well, actually it does matter to us but we're happy to be here now with [baby]".