I think I've found two bugs:
1. Whores pregnancy doesn't check if she was already pregnant:
<<if !$slaves[_i].gender && setup.percentageChance(5)>>
<<set $slaves[_i].pregnancy = 0>>
<<set $slaves[_i].pregnancy_father = 'unknown'>>
<</if>>
and
<<if !$guests[_guestI].gender && setup.percentageChance(5)>>
<<set $guests[_guestI].pregnancy = 0>>
<<set $guests[_guestI].pregnancy_father = 'unknown'>>
<</if>>
So existing pregnancy is overridden if the chance passes.
I think it's probably better to use usual setup.pregnancyChance function - it checks if girl is pregnant, if she's a breeder the chance is higher, and it kinda makes sense to have same chances as in all other sexual encounters.
2. Offering a girl slave to a boy slave doesn't check if the girl is a virgin, so she remains a virgin if she was initially. She still can get pregnant, though
