I don't know about only being allergic to males sperm though. You need sperm to make girl babies too.

Could be just statistics, every baby has .5 chance of being a boy and .5 chance of being a girl. Or, maybe they are more in the mood couple days before the woman ovulates each month (girl sperm last longer).

4 girl babies in a row isn't statistically 'out there' enough to make a call like that without other evidence.