Babe, your guy gave u his reason-SEX. I cannot crusify the guy until you furnish me with some necessary informations.
1. Do you people have sex? If yes. Two things are likely to be responsible for his unfaithfullness.
a.May be you're not very good in bed. Suggestion: you can do better in bed, just try and improve.
b.He is a cheat. Suggestion: better run for your dear life before it'll be late. Broken engagement is better than broken marriage.
2.If you guys don't have sex, i mean, if the relationship is still pure. Quit, you have nothing to loose. Just thank God for revealing to you a future danger. Personally i don't believe in ''HE WILL CHANGE FOR GOOD''.
Anything you know that you can't tolerate in future, don't just try it now.
This is my own opinion. Most importantly, seek God's advice.
Thanks