Now, you probably break up with your boyfriend.
Polyamory is really, really hard to pull off and keep a stable relationship. I know people who manage it, but A) they have a strong relationship in the first place and B) the people either partner sleeps with go through a kind of vetting process where the couple meets them and mutually agrees "Yeah, they're worth boning".
Realistically speaking, if you went off and had sex with another guy, and now all you're doing is thinking about the other guy, the days were probably numbered for your relationship in the first place. Sit him down, lay it out, and be ready for the consequences.