Is it wrong that I don’t want my boyfriend seeing his ex secretly?

Ok so is it wrong for me to not want my boyfriend to see his ex.
He was in love with this girl for 2 years.
And I’ve recently found out that he’s been seeing her behind my back, and even once when I was with him (his family has another house we occasionally visit, and she lives near there). I wouldn’t mind it as much if he’d have told me, but he chose not to. She calls him in the middle of the night as well. I don't know how to go about it.
Is it wrong that I don’t want my boyfriend seeing his ex secretly?
Post Opinion