Why does he ask if Im talking to?

It means he is jealous, and suspicious, and insecure, and you should move on. Nothing you ever say or do is going to make someone who distrusts trust. If he asked once, thats a signal that hed like a more exclusive relationship, but asking often is a sign of trouble. GTFO.

What to do if shes talking to other guys?

If she is talking to other guys, dont try to tell her to stop talking to them. She knows her boundaries. Trying to control her actions will only make you the bad guy and she will lose respect for you. By controlling her, you will just portray yourself as an obsessive boyfriend.