Is it normal that my boyfriends friends have this joke?

My BF and his mates have this joke where every time I say hi to one of them, they slip there fingers into my underwear and flick there fingers at my pussy. We're really close and have seen each other naked so I don't really care, but I don't get the joke and I feel kinda left out. The flick my pussy and say: got to come to cum in yer pussy don't ya, girlie? I seriously don't get it! Please help!