cheating ok?

soooo my boyfriend keeps on hooking up with his ex ‘behind my back’ (tho he doesn’t make it secret and she tells everybody) yet when i go to break up with him he constantly threatens suicide and makes my life a misery to the point where it’s just easier to stay with him even though i want to see other people. am i justified in sleeping with somebody else in this situation? i kind of think of it as having an open relationship i guess only difference is i know if i were to broach the topic of an open relationship he’d flip.

his mentality is kind of like he can do it but i’m not allowed to with all sorts of things (stole a bunch of tabs from me once because he wanted to do E and he didn’t like the fact that i’d already done E before or was going to again)