Wow dude, wow. How absurd. First of all, the adultery analogy is simply a non-starter. There is no legitimate marriage with adultery. There is legitimate marriage with birth control, since most who practice it have also had children. Epic fail on that one.
Also absurd is the notion that within marriage, that sex should always bring with it the possibility of conception. What about those who've had multiple kids and don't want more - and thus have surgically made the decision not to? What about post menopausal sex? Are these not legitimate? That's just ridiculous.
Do you agree that pornography is adultery ?