Can you be friends with ex boyfriends, girlfriend.

So I'm not sure why it makes me feel weird to write this. But many people hate their ex boyfriend or/and boyfriend current girlfriend. Last night I went out for dinner with my exs girlfriend. I'm still friends with him because of my brother (he was friends with him before we dated when we were younger) and she invited us out for dinner for her birthday. I ended up giving her a beautiful gift. And ( when I had my baby girl she gave me flowers and some baby clothes. I don't understand why women feel the need to be jealous over something like this, am I the weird one or should I just not be friends with her? 😂😂. I I'm so lost at the moment because people find it weird. Like why tear women down, when the past is the past. I'll edit if I get questions. 💋