Most Helpful Guy
As a married man, I can say that a relationship pretty much rules your life. Actually, it doesn't until it stops working and believe me, her attempts to make me acutely aware that it isn't might just as well fill up my whole life until I deal with it. Don't worry, I do the same :)
So whoever said the sentence in your question wants to pretty much cheat fate and somehow be the exception. Well, there is only one way to do that, but its not really through a 'relationship', since Its impossible to keep a relationship low-key if it keeps functioning -- they just keep growing infinitely (but asymtotically) or until they break.
Being an FWB on the other hand... well, that is possible because the 'relationship' component is removed altogether. I don't really encourage people to do FWBs...
1THIS IS NOT RELEVANT ANYMORE