Polyamory is a real thing -- not just something you saw on an episode of Big Love. And whatever your beliefs on the morality of it, people practice it. Happily. Polyamory is the practice of, or desire for, intimate relationships involving more than two people, with the knowledge and consent of everyone involved. (No, thank you, Wikipedia.)
In today’s world of blurred sexual-norm lines, polyamory is becoming more and more popular, or at least more talked about. If the guy you're dating is in this type of relationship, you have to ask yourself if you are comfortable sharing. If you are, then you go out and have your own fun as well. But you have to be honest with yourself and ask if somewhere down the way your proclivity for monogamy will cause problems. Chances are, he's not going to change his mind and "pick" you.
If the woman he's living with has no idea, then yes... he's a scumbag. You need to get out of that situation 30 seconds ago.
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