Faithfulness is very important to me. The only time infedelity is acceptable, in my opinion is when both parners are open about it and in agreement with each other about it.
To me sleeping with another person is worse than loving another person. This is because you can't control how you feel about someone even if it's not your spouse, you don't have much control over your emotions...you can repress or hide those feelings of love but you can't get rid of those emotions. Someone may not even be actually going out with the other person they love but still have those feelings and be in a marriage without it considered cheating. just like when you're depressed, you can act happy and wear a fake smile but inside you still feel that saddness, it doesn't mean you want to, but it is just there. people fall in and out of love and it's completely natural.
sleeping with someone, however, is a whole different story. it is physical and often based on infatuation. you have self-control. you can't stop yourself from having feelings of love but you CAN stop yourself from having sexual relations with another person no matter how strong the desire. it is an act that you can never take back (but you CAN fall out of love) and it is a CHOICE that you make (you can't choose who you'll fall in love with). so I feel this is much worse when it comes to being unfaithful.