I would say that Nate does sometimes cross the line into sexualization. I do agree that, on its own, shirtless and muscular guys aren't by default a sex object, just as women who wear low cut blouses etc are not by default sex objects. Nate does periodically blur those lines. Not a problem, and it's tame enough that it's no worse than anyone would see on a PG rated show, etc.
Also, something I think we need to take into consideration is his age. The sexuality spectrum has always existed, but it's relatively recent where it was more defined. In addition, Nate came of age in an era where sexuality and corresponding labeling/categorization has quickly fallen out of favor. Fewer young adults feel comfortable saying they are straight or gay or bi.
When I (a 47 year old man) grew up, the older generation scoffed at the idea of bisexuality. It was looked at as a gateway mode to being gay. However, my generation came to better understand and appreciate bisexuality, but again we were shoehorning people into three distinct options.
Now we've surpassed even that, where today's young adults basically just do what feels good and right in the moment, sexually, and enter into partnerships with whomever most appeals to them. I'm not saying the social stigma is gone completely, but it's definitely fading and, most likely, will be completely gone in a generation or two.