All the relationship advice around agrees that it is perfectly fine to date a divorced man. Sure, a divorce happens when two people agree they cannot be together anymore, but you are not his ex, and its entirely possible that you can have long term relationships with this man and maybe even marry him.

Is it a sin to marry after divorce?

No. Although we may want to personally exercise “grace” and say remarriage after divorce is not a sin, the Bible clearly calls remarriage after divorce a sin because marriage only ends in death, not in divorce. We cannot condone what God clearly calls sin (Romans 1:32, Isaiah 5:20).