HI DragonPie, sorry I'm kinda confused about what you mean by "queer community" do you mean LGBT or something else? I'm lesbian and it's pretty common in my local LGBT community for people to be in open relationships, or be single and dating multiple people at the same time, so if someone wants monogamy they are expected to ask for it directly, and not assume it. So if your friend had some expectation or assumption of monogamy from you, I feel like it was her responsibility to mention that directly, and not ignore the topic and then get upset when you had sex with someone else. That's not a good sign for how easy it's going to be for the two of you to communicate about important issues, seems to me.
I don't mean to oversimplify what is probably a really complicated situation, but if she's not ok with you being poly, and you're not ok being mono, sounds like this isn't going to work. If you might be agreeable to a mono relationship with the right person, that's one thing, but deciding to go mono to please one specific person seems like not the best idea. yes it's necessary to make sacrifices for any relationship, but starting out by compromising on a core believe or live strategy seems like getting off to a bad start.
sorry to be so gloomy, I just got out of a year-long relationship that ended due to incompatibilities that were there from the beginning, I was just too caught up in the romance to realize it. Don't want you to make the same mistake.