For me this was the culmination of a long held desire to have a 'gender-free' relationship, whereby either partner could take any role according to the mood of the moment. I don't like behaving the way society expects me to, in or out of bed.
To answer the question of 'why do guys like being penetrated', you have to look around the board - there are many different reasons given by both men and women here (and elsewhere) as to why they like being anally penetrated.
Younger members may not be aware that here have been some fundamental shifts in western society's ideas about gender roles in the last 50 years. I sometimes wonder whether firstly, anal sex is some kind of 'anti vagina' movement - vaginas were a strictly taboo subject back then, whereas now they seem to be pervasive, one way or another. And secondly, whether guys want to be penetrated because they want to have some of the fun that women so obviously enjoy.
Feel free to shoot me down
