I have a 20 year old son. If he was dating a girl and she cheated on him, I would be very concerned if he dated her again. Cheating shows a serious disregard for someone.
If someone cheats and they've been married a while and have children, I think it is worth the effort to try to forgive and forget. But in your case, I know your family thinks (and they are right) that you can do better. Cheating prior to marriage in NOT a good sign for the future. I don't know you guys, but I want the best for you...a cheater is a red flag!