I still say that favour given simply because you'll get something in return is wrong.
I was watching an episode of House a few days ago (a doctor who diagnoses rare and deadly illnesses), and in one episode the main characters were lesbians. They had the same blood type, and the girl who was sick needed a kidney transplant (I believe) or else she'd die before they could properly diagnose her. Her girlfriend gave up her kidney, but the main ethical issue was that apparently the sick one was going to dump the other.
One of the doctors though it was highly unethical to not tell the dumpee, that she was going to be dumped before she gave up her kidney.
My main thought was that if your love was superficial enough that you would only save your Love's life if they promise to stay with you was bascially "Blaspheme!"
As I told Grimwell, "If you were dying and needed my bone marrow or something, but told me before hand that you were going to divorce me

afterwards, I'd still save you and give you the marrow. I love you, and my love is not that superficial. Trust me, you'll live if only so that I can punish you fully later.

"
Trying to create a world, even in words, is good occupational therapy for lunatics who think they're God, and an excellent argument for Polytheism. -S.M. Stirling
http://www.bamatthews.comThe Writings and Musings of B.A. Matthews