Classic rule of cheaters: if he cheats with you, he will cheat on you. 100%. Which is very deserving for the mistresses. You ruin someone else's life, you deserve to have your life ruined in the same manner too. Don't cool yourself.
He may change for the girl. He may also not. A lot of people will say "oh he's a cheater absolutely" but often times in marriage cheating happens because well they married the wrong person and they no longer like that person. It's a lot harder to back out of a marriage. Like in any old relationship, you can back out at anytime, barring some other issues such as family and living together. Marriages however are a lot more difficult to back out of, and this can lead to cheating.