Not so fast!
Here's the skinny on relationships with SPs.
1. The outcomes are no different than in conventional relationships (the socially acceptable standard), other than what you suggested - everything is out front. Wish more conventional relationships started out that way, but our primordial possessive impulses preclude this from happening, particularly in the tender years.
2. It follows then that, if this type of relationship is, arguably, considered open, then what does that say about conventional relationships? Are they closed? And in what way? The old adage that people have three distinct lives (i.e. public, private and secret) is more likely to apply to closed or conventional relationships. At least, most posters in here who are concealing their pooning from their SO would certainly fit into that category...
Granted this topic has been discussed to death. But new posters who are unfamiliar with the Board don't know that. And, many threads that have contained lots of discussion on the topic have been deleted, or broken up by the departure of posters. New posters may have something to add that hasn't been said, and since this an obsessive and recurrent topic, we can either leave the interested parties to play it out among themselves or let the thread wither on the vine.