Speaking as one of the well-endowed women (size DD) I've exprienced the same problems during sex. My husband will lean down and try to play with them and slip out. I have two problems actually: 1) my breasts are too big to really play with, and 2) my breasts are usually too sensitive or not sensitive enough so that it isn't all that much fun to have my husband play with my breasts.
I envy women who have smaller breasts. You don't have pain or strain on your back, can wear cute shirts that they don't make for larger breasted women, and can run or play sports without it hurting.
But while I envy smaller breasted women, I wouldn't trade mine just because my husband has trouble playing with them during sex. There are lots of other things to explore at that time too.
uptown- it sounds to me like this is more of an issue for you. Talk to your boyfriend, experiment, and hopefully it works out. I have the feeling that if you leave the relationship just because of this issue in search of a guy who doesn't love big breasts, your problem won't be solved.