Rex's nipples and chest are very sensitive and he is extremely self-conscious about it, to the point that he hasn't exactly felt comfortable to admitting to previous partners that this is a huge turn on. Something about enjoying it makes him feel incredibly emasculated so he's shy to try it with others, but is likely to let his curiosity get the best of him. He's also far from his home realm of terrifying, tank-esque superheroes, so the freedom in this place makes him even more desperate to try it.