Well, from a female perspective, I have never tried to "get with another person" while in a relationship. No, I think that if I am interested in someone else, I owe it to the person I am with to break it off with them first before moving on. That is a respect issue. However, I did steal another girls boyfriend on purpose though. She stole my deceased grandmothers ring she had given me before she passed, so I stole her boyfriend. Not exactly a nice thing to do, but at the time I didn't really think about being nice. I also stole a bride on her wedding day once, but that is another matter, as I just convinced her not to go through with it.
I have had ample opportunity to steal other women's men, but why would I want their leftovers? I have always refused to date anyone that has ever even dated any of my friends. To me, my friend is worth more to me than that, so of course I would not even date someone they dated like many years ago, that just seems off limits since, as a friend, We have a "unwritten contract" that allows us to complain about all relationships past and present, and that would be infringing upon that.