When I think about defining a boyfriend, I guess I'd define that as someone with an agreed upon commitment to continue seeing one another with a promise to, if things should end, have a conversation about it ending, along with reciprocated feelings that we are interested in long-term pursuit of a relationship with an end goal of cohabitation. It feels very clinical to think of things in that way, though, and also very black-and-white. Whether that relationship is monogamous or polyamorous is really up to the couple, or triple, or even more!
I have a preference for monogamy, though, and would rather have a relationship with just the one person than one with others. I also don't really like open relationships.
I've had MANY short-term partners and a few long-term relationships. If I were to consider, say, six months as a "serious" relationship, then I've had four of those.
I've also recently had a three-month long-distance relationship but we broke up in January. I'm currently dating around a bit... there are two guys I've had two dates with and have a third date scheduled with one of them tonight and a third date this weekend with the other. It seems it's time to pick one to pursue, and I rather like this introverted man named Ian who is pursuing his PhD nearby. I think we've got more sexual and romantic chemistry. The other man, Randy, has an established career and a lot of outgoing energy which I also like. One's ENTP and one's INFJ, both of which work well with my personality type (ENTP), but I've always really liked those INFJs.