Why would an ex be protective of whom I am dating when he is in relationship?

[Deimy]

[TroyDT2]

A couple of possibilities:
1. Just because the romance failed with you, he still might think you are great and deserve someone who will treat you right.
2. He's want to keep you in reserve in case his current relationship fails.

These both assume you have no children together. If you do, then the reason becomes the protection of the child (ren).

[Deimy]

He was very harsh on me after break up and quickly jumped to other relationship and already lives with her just after 4 months together

[Kaneki05]

I am assuming you two are still friends? Then that be the reason. If you weren't talking to him then he couldn't be. But of course a friend will be protective even more so if an Ex.

[Deimy]

He was very harsh on me after break up and quickly jumped to other relationship and already lives with her just after 4 months together

[Guy13]

He’s playing with your head

[Andres77]

i'd only see it as justifiable if you were dating scumbags and he still cared about you. otherwise it seems jealousy is getting the better of him and he's being manipulative.

[Berethor]

Because men tend to be territorial about women that they owned.

[MadClown]

He still didn’t sell all his stocks (you).