All prestige classes work the same way (unless otherwise stated in an "ex-thingie" paragraph for the "thingie" prestige class): when you lose a requirement, you lose all special abilities granted by this class (but you keep BAB, saves, HD, hp and skills).
So, a female-only prestige class member who somehow change sex lose all his formerly-her special power.
Similarly, an Eunuch Warlock who get the benefit of a Regeneration effect to regain his malehood lose his EW special powers (Mighty Spells and bonus slots -- these things sucks nonetheless, I always found that class' only purpose is to create villains far below their CR).
And doh!, I completely forgot the Rashemen witch class from the FRCS. Note, however, that these witches comes from Rashemen, not from Rashewomen, so it should balance out... (What, my sense of humor is so bad it could frighten a troll ? But it do !)
For ways of getting a female PC pregnants against her will, look at Shark's thread entitled "Madness, Rape, Slavery, and War !"...
Otherwise, even without pills or condoms, it is possible for a girl to enjoy boys without risking getting pregnant (although this require practices that could be banned and seen with disgust in some cultures).
KaeYoss, maybe that virgin-only PrC you saw was the Shield Maiden from Asgard 5 or 6 ? I personally found such a requirement makes it a class totally unsuited for PCs, unless it is precised powers are not lost. Otherwise, she's screwed if she's screwed (I should be ashamed by such a bad pun, I really should). By the way, someone was talking about unicorns: they make poor cohorts for the same reason.
Moe Ronalds, that's "chauvinist".
Dragongirl:
No self respecting woman allows herself to be seduced in 5 minutes. IMO
Not all women are self-respecting, however.
Aww come on, there is more to seducing a man other than making him rise to the bait, so to speak.
This depends on the man in question, I'm afraid.
And so, beholders (I, Tyrant = Eye Tyrant: their humor is even worse than mine) don't lay eggs ? I was sure they did. Strange. Must have to do with beholder hivemothers and hive cities. Well, at least it's not something foul like the mind flayer's way of reproduction.
Is there any feat or class with a requirement of hermaphrodism or genderlessness, by the way ?
We also have the Song Dragon (from Monsters of Faerûn, formerly known in 2e as weredragons) who, when polymorphing into human form, are always female, regardless of the actual dragon's sex. It must be a bit weird for the male song dragon, to switch sex this way. I even wonder if he could get pregnant in such a form (and give birth to a half-dragon).