Mr Flinstone, your "rationalisations" are not very rational.
Of course she's awkward at YOUR family gathering, especially if the people attending are anything like you - loud, intense and dumb (not trying to be a dick here...).
For a lot of people a situation like that would be extremely draining (especially if introverted) and it's not strange if one were to withdraw.
Regarding the sex part, you don't strike me as the most hygienic guy and one explanation as to why your GF may not want to have sex with you is that you're a slob... and also very selfish. Not an attractive combo.
With that said, I think it's good that you broke up - mostly on your girlfriends behalf but also on your as you don't seem mature enough for a serious relationship.
Not sure if that helps but I hope it does.
