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LW, get out as soon as possible. This guy is controlling, manipulative, and even if he gets over this threesome thing somehow, he'll find other ways you don't measure up, and torment you for those.
Alex, you keep talking about how a world where people don't care about their partners' previous exploits, including threesomes, is an ideal world where people follow idealist "feminist" rules, and that world doesn't really exist.
(1) Maybe the problem is that you're a grad student at Brigham Young University or the like, and in your world, you may be absolutely right. But in a lot of other worlds, people genuinely don't care.
(2) You paint the scary image of imagining your girlfriend being penetrated by two men --- which just shows your insecurity and unmasks what's really going on here, which is jealousy and ... insecurity.
(3) You look for strength in numbers by trying to say that all "normal men" think like you, and if they claim otherwise, they're pretty much lying to please their feminist overlords. If that thought helps you cope with your world in which threesomes are not allowed, then godspeed. Just know that you're wrong. :)