Even an evolutionary view of sex makes it very clear how the anatomy (i.e., and thus the sexes) is/are supposed to be engaged.  Expression of love and fun has no bearing on evolution, except with respect to how love and fun affect survival of the species.  e.g., Sex is fun because it promotes sex and, therefore, procreation; sex establishes bonds between the partners to assure that the offspring have two parents to raise them and thus increase the chance of survival. 

I'm not sure given your perspective about homosexuality how you could argue for any sexual boundaries.  Any person that comes up with a good rationalization for a sexual perversion that someone disagrees with is another scapegoat.

Loving God requires paying attention to what God has made very clear what he wants from us; it does not require accounting for what makes us happy or comfortable.