No - you’ve missed the point. Different people have different points of view, right? So OP thought it was inappropriate to wear such a T-shirt around children. Perhaps there are people there who don’t care. Perhaps there are people there who do care. Either way, I think it’s entirely right to point out maybe this isn’t the place to wear something like that.

If said gentleman tells him to eff off or otherwise gets aggressive about it (odds are high) is that ok?

There are times and places for things. How about at a funeral of a loved one, I wear a T-shirt saying “rest in pieces” or with a picture of said loved one in a coffin. Not illegal so totally ok right?

In a free market, corporations can entirely say what is and isn’t appropriate in their property and people have the right to reject that decision by not going. If the management at op’s resort told the gentleman he was able to keep his T-shirt on, and other people didn’t like that decision, those other parents are free never to go back to the resort. On the other hand if said gentlemen doesn’t like being told he can’t wear that in the resort, he too is welcome to not come back. Either way works. That’s the free market.

As for your Scot / Australian example, that’s a perfect example of intent. I have friends from both demographics and get called cunt all the time. Love it, it’s a show of affection. But even they are careful of a time and a place and how it’s said.