Physical love exists when people are attracted purely in a physical way; they don't invest their emotions in such attractions and are instead driven by their more primal needs to engage in physical affections.
Certainly Tom and Myrtle fall into this category. There is no indication that he truly cares for her, and Myrtle dresses in a way that is intended to catch Tom's eye. Their relationship is purely physical, and they have an apartment away from Daisy where they can be together.
However, the physicality shifts when Myrtle brings up Daisy, and Tom becomes physical with Myrtle in a different way by hitting her, causing her nose to bleed.
This suggests that the bonds of a purely physical love do not run deep. Although Tom and Daisy do not have a relationship that most would admire, they are bound together in ways other than physical love. And as it turns out, neither Myrtle nor Gatsby nor manslaughter can drive a wedge between those two.
Tom and Myrtle's physical relationship is a superficial means of entertainment for Tom. He never intends to grow it into something more meaningful because ultimately, Myrtle is not meaningful to him. She is his ticket to a little break from his wife--nothing more.