Rex has several abilities that are the result of covert government experimentation. While he has always been naturally athletic, limber, and flexible, his skeleton and musculature has been adjusted to make him hyperflexible. This means that he is probably going to be spread eagle-ing, hoisting his legs over his head, and bending and contorting himself into a variety of positions. Being stretched out in more ways than one is also pretty overwhelmingly pleasurable for him, so he has a slight preference for being put in positions like this, but depending on the situation he may feel a little self conscious to admit it, especially around other hypermasculine potential partners.