A standard jibe from proponents of “gay marriage” is that marriages don’t eat each other up, so John and Mary’s can’t be affected by Ron and Barry’s. As one blogger puts it, “I can’t help but feel sorry for people whose families are so fragile as to be destroyed by someone else’s decision to make a long-term commitment to another person.”
The jibe fits very nicely with current understandings of human conduct, which tell us that we are (and should be) independent individuals making up our own minds how we will act based on personal goals and the incentives and disincentives our environment offers. On that understanding, which people consider a matter of simple rationality, the jibe seems unanswerable.