AITAH for ending things with a girl because she was sleeping with other people even though it was before we were 'exclusive'?

I (32m) am fairly happy with the life I've built for myself. My career, hobbies, and platonic relationships are more than enough to keep me happy and I don't actively look for relationships or sexual partners even though I do hope to get married someday. Of course, sometimes people do come into my life and we date or sleep together, but it's not a obsession like I see with most of my friends.

Then I met Helen (28F) at a party through mutual friends and we really hit it off. I asked her out and we had a great first date and ended up sleeping together. Things seemed to move relatively quickly from there. We saw each other about five times in the span of two weeks after our first date, all the while texting and even having an occasional phone call. I won't lie, I felt like I was falling pretty hard for this girl. She was everything I could have imagined wanting in a life partner: intelligent, independent, passionate, and kind (not to mention gorgeous).

Well it all went south last week when she asked me if I was seeing anyone else. I told her of course not and that I don't sleep with multiple people at the same time. But then I wondered about her, and she told me she had been seeing two other guys, but she wanted to be exclusive with me. I asked her if she had been sleeping with them and she said yes. She had even slept with both of them at least once since we started dating.

Once I gathered my thoughts, I told her I didn't want to see her anymore. Here I thought the feeling was reciprocated but I found out I was just one of a few items on the menu for her. She started to get upset and told me that since we hadn't discussed exclusivity, she didn't owe me anything and that it was the whole point of this discussion. I told her that while she didn't owe me it, the fact that she didn't stop seeing those other guys on her own volition showed that either she didn't feel the same way about me as I did about her or we have totally different ideas around sex and intimacy.

She said she had dated so many guys in the past who wouldn't commit so she didn't want to end up alone if I wouldn't be exclusive. I could feel the hurt she had gone through when she said this, but I guess I don't understand, why keep around casual sex partners if you want something serious? If she wasn't willing to take a risk for me and potentially get hurt because of her past experiences, I don't want to play the role of intimacy therapist (I didn't say this part to her though). So I told her my mind was made up and she left in tears.

Anyway, my friends have pretty much dogpiled on me after hearing about it. They told me I was an asshole and brought up a bunch of talking points about feminism and slut shaming. It seemed pretty unfair since I never tried to make Helen feel bad about herself, I just didn't think we were compatible and it seemed better to end in then and there.