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The LW needs to view this situation as a gift. The hardest part of marriage is how you and your spouse deal with the difficult situations life throws at you. It's easy to be in love when everything's going well, but people, emotions, & reactions can vary wildly when things go awry. Most of us aren't tested when we're dating, it's years later when there's kids, a mortgage or other issues that you're responsible for. The LW and her potential spouse (she said they talked about marriage) know now how he handles problems. In this case, a small problem (whether a threesome is morally ethical or not is irrelevant). If he's willing to give up on the relationship because of this issue, he's not husband material. And no, he won't change and be more committed when you're married. He'll be a pain in the ass when you're married. Sorry, but at least you know now and can move on.