I'm female and blame Buffy. I always have done.
From Riley's PoV the relationship broke down because Buffy was more focused on her slaying than she was on him. He thinks that her experiences with Angel, Dracula and Spike mean that she wants a boyfriend that's not so Ordinary Joe. He thinks she isn't giving enough of herself to him and wants to experience that 'darkness' for himself.
Interestingly, this has never been my interpretation of why Riley thought the relationship broke down. I've never seen this apparent issue that Riley had with Buffy's strength nor her role as the Slayer. Personally, Buffy's role as the Slayer has never factored in the breakdown of her relationship, rather its her role as a girlfriend; one that she plays poorly and only how and when it suits her. I think Kean makes an interesting point about the vamp biting stuff, which I would never condone, that it was about a desperate attempt to try and understand what was driving her. A poor decision and bad judgement, of course, but I can understand his reasoning.
I seriously don't take issue with the ultimatum at all. What else was he going to do? Continue to be made a fool of by the woman he loved or move on? If Buffy really had, as she said herself and as has been quoted here, given her heart, body and soul, then all she had to do was actually find it within herself to SHOW it to him. And she couldn't. What other sign did Riley need to understand he was going to be better off leaving?
Riley's insecurities (a term I don't really like to use because I think it makes him sound as if he was being self absorbed when I actually think he had perfect reason to feel the way he did) did not come out of nowhere. He was being continually pushed away. And yes, Buffy had other things to worry about, but as HisMrs (I think) I said, of course he was going to be hurt by the fact that Buffy did not turn to him, did not even tell him her mother was in hospital. This was a huge factor in his decision to leave, it was the first big sign that Buffy really wasn't treating him in the way that he deserved to be. In terms of his vampiric misdemeanour, he went about it the wrong way but his 'silly little' ultimatum was pretty fair.
I've always felt that Buffy treated Riley unfairly and made the whole thing about herself. It was all 'I've done this' 'me, me, me' and she couldn't even take a moment to take stock of what had happened until it was too late. She too chose to make it Riley's problem with her strength and her duties instead of looking within herself to see what role she might have played.
Also, I don't see the blowing up of the crypt as Riley being the 'I have to be in charge' guy. It was one of Riley's finest moments; well thought out and planned and executed with the perfect amount of triumph and cockiness. Perfection.