We are all DEEPLY influenced by the non-stop indoctrination and demands that we stay in our gender lanes. Of course, theoretically, people have every right to choose whatever they want for their relationship but it's awfully hard to believe that it's truly a choice given all the societal pressures. My husband and I found this out first hand when we decided to open up our marriage. We thought we had a pretty egalitarian relationship, but we were soon confronted with all of the ways that we had unconsciously bought into all sorts of gender roles and expectations without even realizing it. It was a bit bumpy, but we worked through it all and are happy and have a stronger relationship for having done so.
I think that humans are incredibly flexible and adaptable but the real barrier is the desire to change (or not) based largely on what other people will think if you do, as well as any power shifts that might result.