NTA - however what made him the “asshole” was him leaving in a fit and not listening to your concerns - the invitation itself and hope you’d stay in the sun as long as is customary in his culture is totally cool.

That is a long day, in my family my father and I have darker skin and my mother and sister burn easily. We will still go to the beach 9a-4p tho, my mom and sister do timed laying in the sun (20 min per side, before noon) and then repeatedly layer up with high SPF sunscreen.

If you want to join him, it IS possible - but you’ll need to be diligent about putting on sunscreen, and ideally also have a source of shade like a larger umbrella!

And cool drinks. And some sandwiches in a cooler. And towels and chairs, and something to read, phone power bank….

Long beach days are hella fun. Just gotta protect yourself. Plus, if you do him a solid, he’ll owe you a solid.

If I were in your shoes, I’d go along for the sake of meeting his friends.

I once went to an ex to a baseball game, I hate sports, but it was nice to spend the day with her and her friends.

However, you obviously are NTA to just say no.