Therefore if my wife wanted to fuck some other poor dude I would stop myself from getting all bent out of shape about it. If she wants to do another chick I will ....deal with that as well.
Then why get married? Stay single and sleep around all you want. Or be swingers, or whatever. But if your SO has sex with someone else, knowing you wouldn't approve, it is a betrayal of trust and commitment. Has nothing to do with jealousy.
Also, it's not just a lie, sex has risks. You are not only betraying trust and commitment, but you are actively putting your SO at risk of disease and pregnancy, or at least having to deal with them.
So, what do we have here -
Betrayal of trust
Betrayal of a commitment (Verging on breech of contract, depending)
Willfully exposing someone to disease
Willfully exposing someone to financial risks (Pregnancy)
Willingly exposing someone to a possible moral dilemma (Abortion)
Fuck that. You wanna fuck around, don't get married or marry a swinger.
Also, I don't wanna sound accusatory, but almost every single person I've ever heard make your argument has had cards they weren't showing. As in they cheated on people one or more times and don't want to feel bad about it.