Hi AK, 

This sounds like it could be very frustrating for you, when you are his partner and might see it happening right in front of you. 

If he is attractive, then this is to be expected, but remember that he has chosen to be with you, not the others who swoon over him. Try to focus on that rather than potential threats to your relationship.

The most important thing is how he reacts to it. If he encourages it and indulges it then try talking to him about how it affects you and perhaps ask him to dial it back or how he would feel if you were actively flirting with other men in his company.   

If he doesn’t then try to ignore the girls who make a pass at him. If they are getting inappropriate then talk to them and ask them to stop or simply show them that you are together by holding his hand or kissing him. They will soon get the message that he is not available.