It's the idea of complementarity that baffles me. I suppose the argument is that some slots and tabs are 'designed' to fit, but some others are not, these being to do with gay sex, even though straight people use similar methods, e.g. anal sex, oral sex. If it's not a design argument, then I can't see the point. After all, people have sex in all kinds of weird and wonderful ways, why should this produce apartheid in marriage?