Why didn't he marry you? If he got you a big rock, then most guys (decent ones, at least) would back off.
On another note - I totally sympathize. My boyfriend always gets frustrated because TONS of guys flirt with me and ask me out. I think when you add guys' fragile self esteems to that, its tough on any guy with a super attractive girlfriend.
Not saying it was right that he broke up with you for that, of course... You can't help your looks.
Find a more confident guy. :)