boyfriend slept with another man

I don't know what to do... me and my partner went on a break we was having a few arguments so decided a brake was best but on this brake he got drunk and started doing coke and then he told me he slept with another man even though he says he isn't gay ... I know he didn't cheat on me as we was on a brake it's not that what's the problem but do I try and save our relationship even though i keep picturing him with a man? To makes things worse I'm 8 months pregnant ... if it was with another woman I think I would of been abit different but how am I meant to know if he won't do it again ? Or that he didn't enjoy it ? I'm so stuck with what to do or with what to say 😢 please no nasty messages just need a few friendly opinions