
I agree to not be a home-wrecker (obviously), but I disagree with putting the blame all on one person. Pretty sure a person can't be a "home-wrecker" unless the other person participates in it. It's kind of like when people blame "the other woman" but forget the guy had a part in it too.
I would hope that a couple would have good enough communication that if something like this was going on (a person actively pursuing your SO), the couple would be able to handle it together and put a stop to it.