Leonardo was a physical and passionate person. He was also dominating, no matter what they did. Despite their age and experience differences, Casey wasn’t surprised Leo took control over their relationship. He made it intense and filled with pleasure. Leo guarded his emotions, sometimes too well. But with sex, the turtle proved to be open. Its how he let go of the control he so desperately clung to. And Casey was happy to be the outlet.
I recommend giving it a read, mostly for the humorous scene with Leo and the Foot.