I’ve been happily in a relationship with my best friend for 3 years & always been honest about the fact I wasn’t mono but nothing happened until last weekend where I met a guy I really liked & drunkenly kissed (a lot). Now the shitty situation: my partner feels betrayed & this other guy wants a serious relationship (I don’t) & doesn’t know I’m poly. I’m new to poly, I don’t know what to do
You say you’re “new to poly,” but I’m hesitant to call this situation polyamory. It’s unclear here what the terms of your relationship with your partner really were. You say you have always been honest about not being mono, but did that extend to making sure your partner understood that you intended to pursue people outside the relationship if and when you got the chance? Or was it framed as just something about yourself rather than an explicit expectation of your relationship?
If you have been in a relationship with someone for 3 years without working this out clearly and explicitly, they may have been under a different impression about the nature of your relationship. In your eyes, you were a poly person who just happened to be only dating one person because no one else was in your life at that time. But in their eyes, was it a monogamous relationship? You cannot rely on assumptions - terms need to be clear and consented to by all parties.
Find out why your partner feels betrayed. You need to be really honest with yourself about what was explicit and what was assumed when you said you “weren’t mono.” 3 years is a long time to let someone assume your relationship is monogamous. It sounds like you needed to do more communicating with your partner before getting intimate with someone else to make sure they understood what you meant by “I’m not mono” and are okay with those relationship terms. If they consider what you did cheating, you need to engage with their feelings on those terms rather than insisting that they implicitly consented by continuing to date you after you said you weren’t mono.
As for the other guy, if he wants a serious relationship and you don’t, then tell him no. If he doesn’t know you’re poly - if you got together with him under the implication that you don’t currently have any other partners - that is a major problem, and in my opinion, a consent violation. But it sounds like your bigger focus right now needs to be on your existing relationship, not figuring out how to say “Hey, I know you think I’m single and you want to date me, but actually I have a long term committed partner and I don’t want to date you, but let’s keep making out because that was fun.”