This kind of thing can indeed make dudes feel pretty awkward. I guess you're lucky that you're not in one of those creepy dojos that uses this very scenario to fondle the women of the dojo (it does happen).
The cold hard (or should it be "warm soft"?) truth of the matter is that if you (as a guy) do martial arts with women, you will occasionally get a handful of boob. Best thing to do (in my mind) is to acknowledge it, laugh about it and get on with training. I think it's usually clear to everyone when that kind of contact is made in all innocence (as a normal part of training) and when someone is taking advantage of the situation.
If someone seems to have a particular problem grabbing you, you might either try to address it with them after class ("Hey, I know you're not trying to fondle me, but if you could actually grab me I'd really appreciate it. I know you might 'miss' and that's a chance I'm willing to take to get to really train. Can we agree for next time?") or you might even bring it up right on the spot, perhaps with some humor to cut the tension, ("they're not glass, I don't think they'll break...").
Good luck, hope that helps.