Flirting - forgive or not?

Só I caught my boyfriend flirting with a girl on Facebook, he recently met with his brothers. She started the conversation and she started flirting but he flirted back and even said "fine. Drinks on me?" And she said "haha I'll think about it" and he goes "what is there to think about ahah?" 
I confronted him via message because he is spending the weekend away with friends (we live together) and he apologised, blocked her, begged for my forgiveness blá blá blá. But all of this via message. 
I don't know what to do. I feel hurt. He has not even called to apologise, only done it via message. I feel hurt mostly because our sex life is not been very good recently because of his stress and because 2 days ago he said "óh when you get a thinner belly, you should get a belly ring." That hurt me like shit and he knows it and apologised afterwards but I don't know. 
I know he loves me and he has done só much for me, sacrificed só much. But I don't know what to do. I'm so hurt, I know the flirting for most of you will probably not be that bad but for me it's almost like cheating. And next year we won't be living together as we have work in different cities (4 hours from each other) but agreed to see each other every weekend but I don't know. I feel like I can't trust him anymore. But I'm not ready to lose him, and I don't think I ever will. 
Any opinions? I'm sorry for the long post but I have no one to talk with. Also he has been messaging me but I have been ignoring most of it and replying to some of it, what do I do? He gets home on Monday.