My boyfriend's crush has a crush on me

Hello! My bf is newly poly and he's made a new friend. He has developed a crush on her, but she's bi with a stronger preference to women (for personal reasons). I feel caught because she says she likes both of us, but likes me more. My bf asked to read her messages she sent to me and now he's upset because of her having a bigger crush on me.

First off, you're not obligated to let your boyfriend see something private, even if he asks you to. This woman didn't consent to have her private conversation with you shared with your boyfriend - next time, it's okay to tell your boyfriend "I'm not comfortable showing this to you, at least not until I get Ermanda's permission to share it." If he continues to push or pressure you, he's not being safe or respectful.

As for this current situation, it's okay that he's hurt by hearing that someone he has a crush on doesn't like him as much as he likes her - but that's not anyone's fault. There's a big difference between being upset about a situation, and being upset with someone. No one wronged him - you didn't 'steal' her affections from him, and she didn't choose to feel this way. He's not entitled to have his crush reciprocated (no one is). 

Now is a time for clear, open, and honest communication. Talk to her about this first - "hey, you said that you like both me and my boyfriend, but that you like me more. Can you tell me more about what that means? Do you want to date both of us, but with different boundaries and emotional expectations? Do you want to date just me? Or is this a situation that you'd prefer not to engage with in any sexual or romantic way?"

Then, talk to your boyfriend - lay out how things are and give him space to respond. If you say "Ermanda wants to date both of us, but she is feeling more explicitly sexually attracted to me, so she might be more physical with me than with you, because relationships and needs do differ between individuals," hey may say "I'm not okay with that." He's not obligated to accept the terms that someone else lays out - but he's also not entitled to set his own terms, like demanding that Ermanda behave or feel in a certain way. 

It may be that the healthiest thing for you three is for none of you to date, and to accept that sometimes, crushes can't be acted on because it's not going to work out. It may be that you two want to start seeing each other, which bums out your boyfriend because it's rough to see your partner date your crush, but he decides to work on acceptance because it's happy and healthy for the two of you. Whatever you decide, remember to keep the focus on agency, respect, and honest for everyone.