Well, I fear you may be looking in the wrong place (although I may be surprised).
This is a forum about polyamory, which is usually focused on the romantic relationship aspect, rather than the sexual aspect (although it's not without sometimes vast quantities of sex).
The normal terms I hear is that if the relationship is primarily about sex and is not expected to, or has rules in place to stop any sort of romantic developments, this is termed "Swinging".
There are many resources out there for folks who want to pursue those types of relationships, and they can probably much better answer the questions that you have.
So have I misunderstood you? How would what you describe be different from swinging?
"Listen, or your tongue will make you deaf." - Native American Proverb