No.  The only thing that could conceivably allow you to break the lease would be a breach by your landlord.  Your and your roommate's conduct vis-a-vis each other has nothing to do with the lease.  I'm going to assume you and your roommate don't have any sort of written roommate agreement.

This isn't really a legal issue.  You need to discuss the situation with your roommate.  Maybe you ask him to get rid of his girlfriend.  Maybe you offer to help him find a new roommate to take your place.  Maybe you'll have to suck it up and deal with it.  In the future, if your situation is such that you need to have a roommate, sign at least a basic agreement that governs your obligations vis-a-vis each other.  I'm sure you can find some samples by googling.