We all live once on this earth. Why would you want to deny happiness for someone in their one life. As long as they're not hurting someone else. Problem solved...
And whatever your moral standing ground is on this subject doesn't matter to the two people who want to get married. How will it affect you, if they decide to be together? If you think about it, does it really matter? I don't think so. Let them get married legally. The only real reason for denying this is for tax perposes.