Women are hitting on my man and I don't know what to do

Me and my partner have been dating for two years now. He's very handsome and I would consider myself quite pretty as well. Ever since we started dating women have been trying to hit on him in front of me. My partner is extremely loyal so in these situations he doesn't do as much as even glance the women so I know it's not because of his actions. In public we are quite affectionate: always holding hands and touching each other. What's interspersed is, that my partner has told me that no woman has ever acted the way they are acting unless he is in my company. In fact he has told me, that women don't hit on him when he is alone.

So I have couple of questions here
1 I would like to get some advice on how to act in a situation where someone tries to hit on my partner? Usually I don't do anything, but I would like to know how to mark my territory

2 Second question is for women who are interested in the guy in a couple: why would you try to hit on a guy who's already holding hands with their gf?