I asked to be exclusive and he immediately cheated

I’m going to be rather challenging here I’m afraid... 

I think what you’ve not taken into account at all is how new to being ‘exclusive’ you each were. I get you were friends before deciding to become an item but it was a very tall ‘ask’ and consequent expectation to go exclusive after such a very, very short space of time. Yes, you’d known him a while and yes, you decided you wanted it to be just the two of you but you made that decision and asked him to abide by that when you’d only been together, if my maths is correct, for 3.5 weeks....  

You say it made sense to you because of previous conversations you’d had but honestly, to expect someone you hardly knew as a partner to reach the same decision as you and stick to it, when they may have not been as sure as you thought you heard probably accounts for his messing around with someone else. 

You have to ask yourself if it really was a good idea to ask for exclusivity once he’d gone on his travels. You’d both discussed wanting to be in a relationship together before he left, but to be honest, I can imagine it might have felt very pressurising to get the call asking for exclusivity, whilst not being completely sure that was what he wanted to commit to then and there. People often agree to things when they feel under pressure or unsure how to say no without hurting another’s feelings. 

Taking a different view, perhaps at the time he meant it and yet because you were so very new as a couple, those foundations of trust, exclusivity were insufficiently bedded down so it when an opportunity came up – whether via booze or not, he took it. 

I think you need to look past all this. Yes, he lied to you (rarely a good look) but basically and from what you tell me, you simply had a different view of what exclusivity meant. The agreement to ‘go exclusive’ was rushed and whilst you had seemingly made up your mind he’s ‘the one’ for you, it was all just too much for him. 

Now of course, he’s aware how heartbroken you feel and no doubt regrets that his actions may have contributed to this, but really, I would say this sits more with you than with him. He’s explained he had doubts even though he agreed you’d be a couple together. You pushed the idea of exclusivity because it made sense to you, so maybe next time, or going forward with this partner, you’ll take a little more time to really understand where each of you actually ‘is’ with things. We live in a very ‘fast’ society – everything all at once if you like. Sometimes though slowing things down can reap real rewards.