And if a person drinks a polyjuice and pretty much becomes an animal, could they be impregnated by an animal of the species and give birth to those things? :blink:
I think you're over-analyzing the world of Harry Potter. J.K didn't make everything entirely cohesive, she wrote these novels for children. The concept of gender fudge is just supposed to be funny, a guy becoming a girl in jest temporarily and then moving on.
Plot-holes and things that don't entirely make sense are inherit in the universe J.K created, so we have to deal with them, ignore them, and move on; we have to choose not to look at the practicality of magic and how it actually makes no sense but rather just looking at the general concepts, and overall meanings, then simply put them to use.
To answer your question(s) more directly, I'd be forced to wonder why any person who knew they were male would take gender fudge, become female, and then have sex with somebody as a female. Even supposing the gender fudge intake was accidental/unknowing, I'm sure it's reversible via other magic (Hospital Wing/St. Mungo's etc.). So it'd take some kind of pervert/twisted/confused person to even do that.
Taking on the assumption that the person was raped (a terrible assumption, I know, but such things do happen and it's an easy way to explain the sex while being another gender) I would assume that their body would have produced no sexual gametes for reproduction at that point (sperm, eggs
**) and as such they could not be impregnated or impregnate.
Even if you disagree, I think this is the most logical way to go. Suppose, for a second, that it was a male that became a female and they were impregnated. What happens to that child when the person becomes male again? Would they have to stay hopped up on gender fudge until they give birth just so that they don't digest the baby when they become male again and (surprisingly enough) don't have a uterus any more?
So there you have it.
TL;DR: No.
**As females are (debatably - the theory is now under scientific speculation) born with all the eggs they'll have, and the gender-fudge (wo)man was not born, they wouldn't have eggs.