And sometimes what is not an issue for one couple might be a boundary for another couple - and no one realizes it until you're smack in the middle of it and it's too late.
Honestly I would rather go into a relationship where there are rules from the start, than find myself blindsided by "this doesn't work for me" when I'm already emotionally committed.
When my husband and I moved cross country, we lived with a relative and his wife for 6 months until we got stabilized, found jobs, and figured out what part of town we wanted to live in. I had asked about "house rules" up front and was told that there was no need for "house rules" because we were all adults and we were family and we'd deal with things as they came up. Except that as things came up, no one wanted to deal with them for fear of seeming ungrateful (our side) and resentment and feeling taken advantage of (their side). What were really little issues that could have been avoided by having a set of rules in place up front, wound up seriously damaging a family relationship that took many years to be made whole again.
I can only imagine how much more devastating that has potential to be when extrapolated out to intimate relationships, sex, affection, love and families.
IMO, I'd rather people say "these are the rules" to start with - with the understanding that there is a willingness to adapt and adjust as the relationships develop, than to leave things so open ended. IME, open ended = hurt feelings, anger, resentment, and ultimately, damaged relationships in the long run.
I'm much more comfortable knowing where I stand from the get-go.
IME, of course. Everyone is different.