Watching a stranger bang my partner. Maybe multiple people or in a threesome situation. I wouldn't say I'm a cuck. I think it's just the idea of seeing my partner looking like an absolutely depraved slut that gets me going and a bit of a cum fetish.

In reality I would be worried about getting jealous or them getting too into it just for me to realize later that it isn't for me.

I feel like I recall seeing a subreddit a while ago specifically for that.