All organisms that sexually reproduce exhibit behavior that prompts reproductive activities with the opposite sex. why would this form of behavior most likely be naturally selected?

This form of behavior is naturally selected because, biologically speaking, there is no way to reproduce without having both sets of reproductive organs. In homosexual behavior, both sets of organs are not present making it more advantageous to mate with a member of the opposite sex. (Please do not imply this answer in a political context that is not the intent.)