Looking for '76-79 movie

Looking for the title and maybe a link to an old movie that would probably be 1976-79 vintage.

It starts with two guys playing handball. One of the guys starts telling the other about a woman he knew who was a real freak "Mishu" I think he called her, but she wasn't Asian.

I remember she was raised in the south and had scene with her having sex with a relative, who kept saying (in a southern accent) "that feels good, that feels real good".

A few other scenes that I can't remember and the twist in the plot is that the woman turns out to be the other guys wife or girlfriend.

Any help appreciated!