I had sex with my Roommates girlfriend [MF]

This is a story from about 10 years ago back when I was in university. I lived in an apartment a few minutes from the university. I saw an ad on craigslist, it was someone looking for a roommate to split the rent with. That’s how I met my roommate we will call him Tom (not using real names is this story due to privacy reasons.) Tom would regularly bring his girlfriend over with him (let’s call her Suzy.) Suzy and I talked to each other here and there but nothing too much ever came out of those conversations. I didn’t really care too much about her as I had a girlfriend of my own but I broke up with her a few weeks before our affair.