<human>: I've now been with my boyfriend for around 4 months. We were friends for a while before it happened, during which he had a different girlfriend who I didn't know. We realised we both liked each other a lot and he subsequently broke up with his old girlfriend to be with me - I must add this was completely under his own duress and I didn't pressure him in any way. 

When we started dating he said that he would like to be non-exclusive for a while as he hadn't been single for a long time and would like that freedom, and that was fine with me as I understood his feelings. From what he said I assumed he might hook up with someone in a bar from time to time which wouldn't have been an issue. 
However a couple of weeks later he told me he'd slept with his friend the night before - let's call her Mary. I am not close to Mary but know her slightly through mutual friends. I was upset as I know he's kissed Mary before, and worried that he might like her as more than a friend. 

We talked about it and he said sorry for making me upset and that he only likes her as a friend. I tried to forget about it and move on, however at a party the other week Mary told me that she's liked my boyfriend for a while and when they had sex she assumed it would lead to something else. (Note, I don't blame her at all for this as she was a single woman and my boyfriend told her it was fine for them to have sex, I actually think she's a nice person.)

My boyfriend and Mary hang out quite a lot as they are in a play together, and he is going to a party at hers this weekend which I can't attend. Sometimes I worry about something happening when they're together, especially if they're drinking. Is this rational or am I being paranoid? I haven't said anything to my boyfriend yet as I don't want to appear over protective or jealous. Just to clafiry, I'm more bothered by the fact it was with one of his friends than the fact that it happened, since I'd said I was okay with it. Any advice is appreciated.

Can you condense the text into three sentences or less?
<bot>: my boyfriend had sex with his friend while we were dating non-exclusively, I am now worried when they hang out.