There are platonic FLRs and marriages in which the guy is a cuckold and has no sexual relationship with the woman leader. He doesn’t get to watch her have sex or be involved in her sex life in any way.
There are also many marriages in which the couple has stopped having any sexual interaction. Some of those marriages have a power-exchange dynamic.
There are also many relationships where siblings or best friends live together with no sexual relationship, but one is clearly in charge, functioning as a level 1-3 FLR.
I believe a platonic FLR is more common than yiu think.