If a sadomasochistic couple is heterosexual, they already HAVE the right to marry and have children. If you or I found that lifestyle loathsome, there's nothing we can do to stop them from living it. What they do is not illegal, as long as they're both of legal consensual age. The same goes for a heterosexual couple that is into swinging. It's not illegal.

Yet if a committed homosexual couple wants to marry, why is that illegal?