I had to sit and cogitate for a while before responding to this since it hit close to home emotionally. Not in a gay context but in an inter-racial context. You see, this is the same lame, biggoted, pile of bullshit I was handed when I had a black girlfriend in 1972. Both my parents and her parents used that argument for why we shouldn't date, let alone eventually get married and have children! Oh, the humilation they'd be subject to. In the end it was her family's overriding hatred of anything white that made the relationship virtually impossible, and she broke it off.
Of course in my father's case it as his Conservative Absolute that white's should marry white's and black's should marry blacks that allowed me to begin to look much harder at many other things he held as absolutes as well.
As an aside, this came from two VERY religious families. One staunch Lutherans, and the other staunch Southern Baptists. Their "god" must be so proud...