Learn from my experince an ex is an ex for a reason. They will NOT change not matter how much they say they have. My ex asked me in the middle of sex and i told him that i had slept with another guy. there's no need to lie about it you were single. If your ex trys to make you feel like crap about it walk away. If he didn't want to give another guy a chance with you in the firat place he would've made sure not to lose you to begin with.
1st thing, you don't get back together with an ex. 2nd thing, if you do that and you're comfortable with them, don't you feel the need somehow to tell them what you've been up to while they've been gone?