Homewrecker

A noun, usually pejorative, referring to a person who has an affair with another who is in a committed relationship. The outside party is usually aware that their partner is already in a relationship, but not always. The term derives typically from a married couple happily living together, with an affair subsequently wrecking their relationship.

1. I met up with this guy at a bar and I ended up going home with him, woke up to his girlfriend screaming at us in bed. God, I am such a homewrecker!
2. I thought Tracey and I were best friends, but turns out the homewrecker wanted my husband more than I did.