I appreciate the details and the screenshot! That particular bug is primarily what’s messing with the WantedPregnancy, but the overall process would be:
Consent (Sex) > Consent (Pregnancy) + Consent (IncestBreeder), if family = WantedPregnancy
The Pregnancy fetish will increase the chances of their giving pregnancy consent as well as having a number of desired offspring higher than their current offspring. Pregnancy consent does have a chance to reset each birth (as the desired offspring number may have been reached).
If the pregnancy isn’t wanted from the start, there’s a chance of her gaining the WantedPregnancy each day. Incredibly short pregnancies are unlikely to see this change, but if you see a dialogue option relating to her wanting to tell you something, it may be that discussion.