Is it normal that girlfriend likes to control me by my balls. comment

My girlfriend and my sister have grown close. One time when my girlfriend had come over for the night for some drinks and more, they got talking about our childhood.
As stories unfolded, my sister shared the stories where my mom used to put her incharge of me when our parents weren't home. She explained how my mom was proud that she was always able to handle me. What our mom didn't know was that my sister used to kick me in the balls whenever I bothered her.
She would hurt my balls in all possible ways to get me to obey her. She kicked from front and behind, kneed, flicked, punched and uppercut from front and back, what not. She would always try to catch me off-guard. The result being I would fall to the floor while my sister would laugh like crazy and walk away in style.
My girlfriend was delighted to hear the stories. But what happened later is my main concern. Now, my girlfriend likes to hurt my balls. She doesn't hit hard, not even close to how hard my sister hit. But she teases me and controls me by my balls. One grab is enough.
She's not abusive, she just enjoys it. And, well even I have started to enjoy being controlled by her.
It's a bit ironical how she became 'the man' of our relationship by using my balls.
I want to ask is it normal? That I'm liking it and she's enjoying it?