You're right, but most people can't do the distinction properly on emotional level, and unless they actually go through it, have no clue how they react. Once you're sure it works OK 100% of the cases, on both ends, then its probably healthy for relationship.

Anecdote - one of my ex was talked into a threesome with another girl by her ex, thinking how refreshing it would be (and she is not a typical nice girl with a good heart to start with). She couldn't handle it at all, and it contributed to their breakup quite effectively.

Nothing for me, I prefer quality rather than quantity (maybe not the best comparison, but I lack better in this case).