Even tho Rudy was there watching, Sylphie couldn't stop Ghilaine. Even if she was stronger, the pleasure was too much to stop. She would have said no before; she would have stopped her. But after seeing how much pleasure Roxy and Eris felt, it was impossible. Eris said no at the beginning, but I guess beastfolk have a way with words and their body. At this point the only way for Rudy to reclaim his wives its to fuck Ghilaine himself.