This may sound like a flimsy excuse, but I'm honestly not bothered by bending the laws of reality a bit for the game, especially for the sake of fan-service or fetish content. I advertise the game as "adult fantasy" for this very reason: I'm not trying to write a non-fiction. There's plenty of things I could do differently for the sake of realism if I was.
As for the specific subject (Brooke): of course it's a little bit silly, and a clear example of me doing what I described above (sidelining realism for the sake of fantasy). But certainly not that far fetched or impossible. Our world is crazy and weird, and I imagine there's at least a few virgin mothers out there. Also, Brooke has her reasons which will be expounded upon in her next date scene and have already been foreshadowed (heavily religious family, being socially awkward and a loner, winkwink).