I CAME A RIVER AND A HALF.

Gender roles are much more fluid in gay relationships - in many cases, what you said is quite literally the norm: one day one holds more of a dominant position, and the other the submissive/passive role. In some relationships, though, there is a clear and mutual understanding of permanent roles. In general, gender roles are influenced by a lot of things, such as physical stature (taller, more built gays tend to hold dominant positions, while shorter, smaller ones hold the passive), personality (outgoing/introverted aspects), and age. At the end of the day, however, I'd say the gays are just like heterosexual couples. There may be more of a sway towards one role on average, but both take on both roles when the time is necessary (ex. in heterosexual relationships there are many cases where women want to be on top, etc.).

Hope that helps.