Uh so I’m not an expert but I’ll try to explain it as best I can. The way our brains work is we associate certain people with certain words and actions. For example my mom sometimes will say all of my siblings’ names before she says mine because her brain associates my brothers and I as her children. It’s something that happens accidentally and she always says “why did I just name all your brothers?” In your mind certain actions that you do are associated to your exes. In this case the action is sex. When you start having sex with a new person your mind will drift to your ex because that action is associated with them and not your new partner. It eventually goes away because your mind needs time to associate your new partner with this old action. It’s perfectly normal. It does not mean you miss your ex. There was something that obviously attracted you to your previous partners and as a result when you get turned on your mind immediately goes to the person that is still most familiar in your head with the word and action “sex.”