Not sure how you can... I tend to stay away from cliche sayings but once a cheater always one. It would take some really life altering circumstances to be ok with this and in the end it depends on the events that took place... My brother's wife cheated on him. She seemed to change but she refused to quit her job ( a male dominated field) so she didn't change her behavior or her environment so it was bound to happen again and it did... As much as it might turn your world upside down your better off leaving him and finding someone who will respect you enough to let you know when they are not happy with you or can at least man up and say that.

A few signs of cheaters:
1. Their phone--- phones are the gateway of communication if your significant other won't even let you get close to their phone and have a lock on it odds are they'll throw the privacy line at you because they have something to hide.

2. Odd hours going out with friends that they normally wouldn't... since cheaters are liars they will lie to you about their whereabouts

3. Not interested in sex, kissing or any physical contact with you... odds are if they are getting it somewhere else and have that "new love syndrome" they would often not care about you and act distant.

4. Finances... if they are out and about... taking their cheating lover out to dinner, hotels etc... they are going to be spending cash and if you are on a budget like most people are well... liars are gonna lie so look for large amounts of cash that don't add up or them telling you wild stories of where the cash disappeared.

Now these are no way indicative that your significant other is cheating but they are good signs to make you question... and it doesn't cover nymphos or people impulsive sexual desires... I could write a book about those people.