Sexual Vulture

Somebody who makes a move, while the person they are making a move on is clearly weak due to alcohol, distress or sheer unattractiveness, resulting in them being easy. They are therefore preying on the 'scraps' in a similar way to how a vulture hunts

Rob: Look at that girl over there, she's a proper munter!

Matt: Looks like David is making a move on her

Rob: He's such a sexual vulture