You don't have a lot in common and you have been together 7.5 years? I find that a bit strange. As for your lack of sexual attraction of late just speaking from experience but when I have lots sexual attraction for my partners in the past it generally means I am no longer wanting to be in the relationship. That could just be me though. How often did you guys have sex prior to you losing interest?
Perhaps being with someone for basically your whole 20's is making you feel like you haven't quite lived the single life or experienced other people which I guess is normal. Not sure what advice to give you but the only thing I can say is do not marry this girl if you are not 100% about it. And she is probably right, if after 7.5 years it is not progressing to the next stage, or at least the commitment she wants maybe it is time to set each other free.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!