For the cuck one, it’s not so much that they bend over and do nothing, it’s that that actively invite it. That’s the parallel to the fetish - just doing nothing but also not consenting would be adultery. Consenting and facilitating it is what makes someone a cuckold.

The LARP comment when I’ve seen used as a pejorative is mostly in “stolen valour” type situations. For example, ICE officers LARP as real cops, cops LARP as soldiers, etc. It’s use as an insult is to imply that someone is pretending to be/do something they’re not and likely could never aspire to be.