Role reversals in relationships
As stated before I observe people. Its almost a hobby for me. Anyway, I have noticed through talking to couples that they seemingly switch roles after a long time together. Nothing as serious as overall masculinity/femininity obviously, but in other large areas. One thing that I've noticed on multiple occasions is that whomever is more dominant usually becomes more submissive and vice versa. This seems more likely to take place when there is a souring in the relationship and the more submissive person feels trampled/overshadowed/whatever. Then it becomes more an issue of asserting dominance over their partner (rather than being dominant in general). I suspect that after years of being dominant without really having to try the once dominant person inadvertently slips into the submissive role.
Speaking specifically of myself, my wife was brutal when we first got married. I was definitely a lot more passive. I had my wedding rings tossed at me, was cursed at, and was ignored/neglected emotionally, physically and sexually. That was about six years ago, and I'm still married. However I am a lot more aggressive in how I deal with her and she doesn't have nearly as much fire as she once did (attitude-wise).
...one can be sure of nothing until it has already happened...