He's best friends with his exes

Let me explain...I've been with a guy about three months now. Sex is great and we get along really good. He's awesome. I didn't care that he had previously been married (three times) or that he had kids (three). I figure, I'm in my thirties and this is going to be a new normal.

However...his exes are very present in his life. One in particular seems to still carry a thing for him, even if they have been divorced for over seven years and she's married again.

What's a nice way of saying, I'm not comfortable with this?

Note: I have not met his kids since we both agreed it might confuse them since we're still very early in our relationship.