I do appreciate his desire to remain monogomous but I would like more eggs:)!!
I would suggest isolating him for week or so, then returning him to the pen after removing his favorite girl. I have seen this before but it is fairly rare. In the absence of a member of the opposite sex male chelonia tend to be attracted to pretty much anything even remotely shaped like a mate, but they do occasionally become fixated on one, possibly because she is more complient or simply because they prefer her.