Being friends with exes

Is it weird to still be friends with an ex?

I have a partner and a few of my exes I have stayed friends with, I am bisexual and used to go out with a girl ages ago and I am friends with her still.

She has a wife and they adopted 2 kids, I have 4 kids and a partner and we have been out for meals with them etc.

I have another ex girlfriend and she is bisexual as well and she has a partner and 2 kids and we have been out with them as well.

He has ex girlfriends and he is civil with them as when he goes back to where he was brought up and goes out his exes have been there as well. I have met them, they are nice and have been out with them.

I have a friend who is saying how weird that is, saying I couldnt be friends with someone my partner has slept with.