If I contracted with someone so that, under the right conditions, I agree to be their slave, that is not a valid contract. I have neither the right nor the ability to consent to that, nor can the person I contracted with enforce that contract.
Maybe I WANT to be enslaved, because I don't have to worry about income or taxes anymore - that's my owner's problem. Maybe my owner WANTS me to be a slave, because s/he always wanted to experience that relationship. Maybe my slavemaster treats me very well, and that's why I want to be his/her slave.
NONE of that matters. I don't have the ability to consent to such a relationship, nor does the one who wants to enslave me. The Declaration of Independence makes quite clear that our rights are:
(1) inalienable and
(2) endowed by our Creator.
The only theology that holds both of those principles to be true is Christianity. Christianity says that we have rights because: (1) God has rights and (2) we are made in God's image and likeness, so we have a share in the divine rights. No other theological system makes those two statements.
It isn't a question of emotion. No one has the ability to consent to homosexual activity. Period. Because no one has the ability to consent to the act, the act is always rape.