If you fall in love with someone and have a sexual relationship with someone but don't have any children with them and they cheat on you and one day decide that they want to run off with their new partner that is their right.  If they don't want you to contact them anymore than any effort of yours to contact the person you love is considered harassment and you can go to jail for it.  If you do, you are officially the bad guy.  You may feel that you were betrayed and that the person you were dating owes you something but society doesn't look at it that way. 

Supposing we change one aspect of this which is that you were married before your spouse ran off with her new boyfriend.  The laws (in the United States) about harassment still apply.  However, the spouse is now considered guilty of adultery.  What that means has changed in the U.S..   

The laws regarding divorce in the U.S. have changed.  At fault divorce has given way to no fault divorce in one third of the states.