About 15% of my sexual encounters have been in relationships while they fvcked around with me. I suspect many more have had boyfriends and i guess the percentage can be doubled if you include the women that didnt let me know they were cheating.
You cant trust women. Their love is conditional and they are opportunistic.
Yet, that doesnt mean you cant have a relationship, just accept female nature and act accordingly.
Improve your odds by making sure she thinks you are the best that she can get. Make sure she knows you have options and are not afraid of walking away or her leaving at any point.
This is when a woman will be on her best behaviour.