Yeah I totally agree with Tom on this one. Your only real option is to tell her you don't want anything serious and it's not what you want at this stage in life.
Sure, you could potentially be losing out on great sex BUT in the long run it will make life far easier for you and also it saves you from being a twat to her.

Also, you know you can get yourself another woman who wants what you want you've been proving this!

My two cents

Ritz