However, if the bull is not into actually being an involved father to this hypothetical kid, there is a huge problem. The fun is over. It means he would be a "deadbeat dad." The fetishistic pleasure could actually end, as the legal issues began, and the baby was accepted as a human being with rights for care and support.
So, if Grant actually did get Lena pregnant, he does not agree to actually being an involved father to his biological child. He has turned over the rights to his biological child to you, David? Lena would keep the pregnancy, not terminate it? If there was a successful birth, are you saying Grant would leave her and the baby? Of course, in no way would Lena ever be "alone." She'd always have you.
You would have to legally adopt Grant's kid to be its legal parent and absolve him of the responsibility of raising the child and supporting it financially. These issues actually are quite pertinent to the MFM poly V. Each V does have to decide who would be in what relationship to any children born from the unions.