Unlike love, relationships are conditional. The rules of a relationship are determined by what each individual is willing to allow in their lives. If you cheat on me, I may still love you but choose to end the relationship because I can no longer trust you. My love for you is not determined by your behavior...our relationship status is.
Despite loving you deeply, I may need to end the relationship because I love me too. Living without someone you love is painful and so is living with someone you love who continuously causes you pain. There are no hard fast rules to relationships. Where one couple will dissolve another may thrive and become stronger. It's an individual decision.
The most important thing to know is that love and relationships are two separate things. Love is spiritual and eternal. It is, most often, uncontrollable. If you keep going back to a bad relationship because you love the person. You will be in that cycle for the rest of your life because true love never ends. Relationships do.