Would you find it strange if one of your exes remained single after the two of you broke up?

Not really, some people just do not want a relationship, others have their reasons as well. Maybe they do not define themselves be being in a relationship and are happily single after having had such a terrible relationship.

My ex took 4 years and he was on a dating site. Twice in a row he reactivated his dating profile on my birthday

No, not strange. Everyone mourns a loss in their own unique way. Whatever loss that it may be.

You are pretty young. Why are still talking about your ex? Their life, their choices.

One of them is still single many years after. By choice.
But so am I.

I'm not concerned with what happens to them after we break up.

Damn. Did you traumatized him so badly, that he won't even date again? He didn't join a seminary, did he?