In my teens I had a rare flash of wisdom and revelation.
I decided that I’d never get involved with a guy who was in a relationship, especially someone who was married or engaged. I figured that anyone who would cheat with me, would cheat ON me.
Not to disparage myself, but I’m OK, not fat, but not model material.
Even if you don’t consider the moral issues. which you should, it’s not worth it to pursue someone who is taken.