Well, just to play devil's advocate, so long as the potion to achieve this allowed the user to become pregnant by her lesbian lovers, that would probably get in without a hitch. It would need some coding adjustments, but not many, mostly around when the PC needs to roll to check if she's been impregnated.
Being able to impregnate your lesbian lovers is another matter; that would have to be handled individually for each character, at least in terms of convincing them to take the potion. Not technically all that much, I admit, but still, it does add to the workload.
The big issue is actually promoting it; namely, there's not a hell of a lot of tribbing, scissoring or other lesbian-focused sex scenes that could believably cause fempreg. Mpreg is easier, it's just straight up anal sex, which tends to come in standard for any character who has a dick. Thing is, the writing team's pretty much all guys, so we're not exactly brimming with the skills to really write quality lesbian stuff.
Of course, there's always the option to get an enchanted strap-on that will let you knock up your lovers. You'd need to write a scene where your lovers can put it on to impregnate you in turn, but even so, that'd be much simpler than the need to fix up the lack of tribbing and scissoring...
Which is kind of a reason to speak against it. We need more lesbian friendly sex scenes, given we're going to have a fair number of female lovers in the game by default.
tldr: fem/fem pregnancy is easily explainable, but we don't got the lesbian sexings needed to make it happen anyway.
