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People make choices. People have values. Perhaps he was attracted to this woman based upon assumptions about her values as ones that might have agreed with his own. Upon finding out he was wrong (considering her past acts) he has every right to question his relationship with her. This does not make him a jerk. If he keeps stringing her along and punishing her too long for something that she didn't do as an act against him, then he is a jerk. But honestly, it sounds more like he really liked her and was upset to find out about all this and now has a serious conflict about how to proceed.
I personally have made choices in situations where I could have participated in threesomes, not to do so. And not because I think it's wrong, but because I don't want to be the kind of person who does(personal value choice). I've had girlfriends who had previously participated in threesomes. It didn't make me dislike them, or not continue to go out with them, but I wouldn't have wanted to marry any of them or get too serious.
Obviously he's a person with different ideas about the world, and they both need to find other people who share their individual values.