If you don't trust him/suspect he's cheating; assume you're right!
Unless you have a history of being paranoid and insecure there's no reason for you to ignore "red flags" or doubt your instincts.
If something doesn't feel right to you it's probably not right for you.
The first question has to be for yourself:
"Is cheating on you a deal breaker?"
If the answer is (yes) it doesn't matter (why) or what his reason is!
Generally speaking the goal of most cheaters is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side.
They're essentially looking to compliment what they have.
Rarely do cheater seek to replace one relationship with another.
Some common reasons given for cheating are boredom or the desire for something "new", their mate no longer does certain things, they feel taken for granted, lack of passion/romance, no longer physically attracted to their mate, no longer have "fun" or flirt with one another, a feeling of missing out especially if they got married very young without much dating experience.
As I stated if cheating is a "deal breaker" it doesn't matter.
If cheating is NOT a "deal breaker" you might suggest couples therapy once you've had discussions with him and have determined he's cheating in response to something rather than just wanting the thrill of being with someone "new".
Another factor is whether or not he's asking for forgiveness and shows genuine contrition for disrespecting the marriage.
Forgiving someone without being asked to is seen as weakness.
Cheating is a decision one makes rather than investing in their marriage usually after they've determined their spouse can't give them what they desire and they refuse to do without it.
Only you can determine what (your) "deal breakers" are.