Rory, my beloved dog is my mother's leg...Any advice?

Rory, my beloved Great Dane, was really getting annoying last night. My mother came to visit and was sitting on the sofa. Suddenly, Rory came (no pun intended) and wrapped his gigantic forearms around my mom's legs, and started thrusting about, in total ecstasy, like he was . He looked like a freaking kangaroo. He had this stupid grin like expression. The worst thing is...I started to laugh hysterically, which made him pump more furiously. My mom was horrified, and tried to push him off, but he was not to be denied. It was terrible...but funny. How do I fix this appalling behaviour?