They need to ask people attracted to men, in general, whether the character they've designed is sexy. And the response is going to be fairly varied. But.
The rule of thumb isn't about what a character -looks- like, so much as the -position- they're in. A sexy pose can make people of any body type look sexy (or like they're trying to be sexy to varying degrees of success) to the people around them
The way a person dances, or smiles, or shakes their hips, or bites their lip... These things are where sexiness comes in. A woman can wear a 3 piece suit and look sexy as heck with practically no skin showing based on her attitude. And a guy can wear a pair of low rider jeans and look outrageously angry and kill the mood.
Similarly, someone in tears and distress, no matter how attractive their bone structure, doesn't look sexy. They look hurt and upset and we approach them on that basis. Y'know?
Skimpy clothes can help frame a situation as sexual, but body language and expression are much more powerful indicators!
Zach Kornfeld in his underwear. A skinny and hairy geek who isn't inherently "Sexy" just existing.
Same guy oozing sexuality at the camera.
He may or may not be significantly more attractive in one image than the other, but his position, expression, and presentation are much sexier in the second image, at least.
And here he is being sexy while wearing more clothes.
Framing does a whooooole lot.