do you think it is wrong to see or talk to your ex when you are taken???

Never wrong to see or talk to one's ex when one is in a relationship. But, it's neither right, too. I'd rather say, it is not wise to do so because whatever you say, it can cause jealousy. In the first place, why do you need to see or talk to your ex? Are you trying to rekindle feelings? Even if for you it's nothing malicious, what about for your ex? You can't blame your present boyfriend if he'd be angry. Try putting yourself in his shoes.

I'm friends with my exes and though my partner isn't totally okay with it, I just make sure that assurance is plenty. There's really nothing wrong if everything is innocent and friendly between you and your ex. But also take into consideration how your boyfriend feels about all this. He's the one you're with after all. It's not so much that they're immature. A lot of people prefer their current boy/girlfriend to not be friends with their exes. It could be that he has had previous experiences with exes or something. Just make sure to give your boyfriend plenty of assurance and make sure that you don't do anything that could drive him crazy and ruin his trust in you. :)