Classic: It's weird, but you've been told their position.

mrsconn23

My brother is 19. He has chronic muscle tenseness, and it sometimes soothes him when his back is rubbed. We’re both home for the holidays and have been spending time with our parents as a family. Thrice now, in public, my mom has, at his request, begun to massage his back.
Under the shirt. For several minutes. She’s done this for years, but now that he’s grown, I think it’s weird to do this in public and in private for that matter. My parents and brother get offended when I mention that museums and restaurants aren’t the best spot for a mom-and-son massage. They say I’m dirty minded and being rude. Am I a prude, or are they committing a social faux pas?

Casadena

I'm with LW that it's super weird to do in public.  But they're not going to stop, obviously. so just remove yourself if it's uncomfortable for you. 

banana468

mrsconn23 said:

My brother is 19. He has chronic muscle tenseness, and it sometimes soothes him when his back is rubbed. We’re both home for the holidays and have been spending time with our parents as a family. Thrice now, in public, my mom has, at his request, begun to massage his back.
Under the shirt. For several minutes. She’s done this for years, but now that he’s grown, I think it’s weird to do this in public and in private for that matter. My parents and brother get offended when I mention that museums and restaurants aren’t the best spot for a mom-and-son massage. They say I’m dirty minded and being rude. Am I a prude, or are they committing a social faux pas?

Brother is likely the younger brother.  Good luck getting them to break the cycle of the baby needing a backrub. 

STARMOON44

Get up and walk out every time. It’s inappropriate. 

short+sassy

It's inappropriate for them to do to do that in public.  Not because they are mother and son.  But because for...anybody.  Edited to add:  And this would be the better argument to use.

However, in private, that's their own business and the LW needs to stay out of it.  Though it would be nice if the mother/son would refrain from it in the LW's presence because they know the LW finds it creepy.  I'd find it creepy too.  Because it is one thing to know logically that there is nothing sexual going on, but it's hard to shake that feeling to see an "under the shirt" massage that is normally only done between romantic partners.