"Yeah, maybe we should just abort, is (however many) weeks too late to abort it for having a penis?"
"Actually i like boys. That's WHY we keep having them." (and look at them like they are complete idiots).
If your older kids are there (i hate it when people are THAT tactless) you could say "do you know you said that out loud and my TWO SONS can hear you?"
We decided not to find out, for a lot of reasons (not having a scan late enough to be very reliable being one) and when people want to know i generally just say i'm not sure, but i did have one crazy exchange whereby a woman asked what it was, i said i don't know, she said did we not want to find out, i said we didn't mind either way, she said but which would i prefer, i said i really didn't care, she said well, but if you COULD choose what would you want, i said really i didn't care at all, she said but i MUST have some preference and i finally snapped and said "Look, i have had FOUR miscarriages, this one can have 2 heads, if it makes it to my arms i'll be grateful, ok?" and she scuttled off back under her rock.