My wife recently asked me this qn too (as i always like to ask her qns )
I told her, I wouldn't break up other relationship. What is the point? If the gal n u together in the end, she might not be together with you forever. It is becos since she could left her ex just for you, she can do that to you when another guy seem to suit her better. Maybe she feel guilty and go back to her ex, you will be alone again. I feel that I wouldn't waste time on such relationship as I see no ending. Worst still, if after marriage, she might sleep with other guy easily as she is not stable in the first place. That's also apply to guy, don't anyhow change, you might get yourself hurt one day.