I think this got it wrong, iirc what happens is that the female orgasm makes the vagina more alkaline when it's mostly acid to stop bacteria from infecting it, and the male sperms are weaker to the acidity than female ones, so if a woman orgasms durign conception it's more probable (probable...
Good ending, their story continues but that's all that we needed to see about it, if they decide to make a sequel later it would be all right, but this works for me.