Yup, as Tamsin said, it's to do with dominance and he is wanting to be dominant over her.
It might also be worth checking his 'bits' are all formed properly. I came across a guy recently who the RSPCA said was neutered but when I did a home visit he was actually pretty deformed around his genitals and on inspection from a vet had two undescended testicles.
Chances are he is neutered and it won't be that complicated, but always worth looking at.