Some people are less prude than others. Unfortunately prudery is on the increase again, that much is true. Luckily, not everyone goes along with that. Swimming trunks for men used to be even smaller a few decades ago than what Sam was wearing. These days, (young) men even tend to be pruder than women... (not Sam though).
And if he was testing the waters: that's not a crime, isn't it? Anyway, i didn't see much reaction. But the girls didn't seem shocked, either. And when you're staying in hostels (which couchsurfers often do if they don't find a couch), you see people of both sexes in underwear all the time. No biggie.
Edit: talking about European hostels & mentality here. The US (with its puritan heritage) might be more conservative in this regard.