All I've heard is that some guys have a preference for one or the other and some couples take turns or 'switch'.
For example, over at www.belamionline.com in the models' bios, one of the interesting stats listed is 'preference' and they'll usually say top, bottom, or 'versatile.'
Maybe I'm the only person who finds this fascinating, so I'll shut up now.
It isn't always that black and white.
Yes, there are some guys who always top and never bottom.
And there are bottoms who don't top.
And believe it or not, there are some gay guys who don't participate in anal sex at all.
Typically there are in between descriptions.
TOP,
TOP/VERSATILE,
VERSATILE,
BOTTOM/VERSATILE,
& BOTTOM.
A straight friend once told me he was a bottom when it came to sex.
I was like "Hunh?"
He said: "well I like to lie on my back and have my girl ride on top of me, doesn't that make me a bottom?"
I explained to him that the physical position doesn't matter.
The definition is about who penetrates who.
Another clever term is "Pitcher vs. Catcher"
Most guys have preferences, but that doesn't mean the lines don't get crossed. For example, two bottoms can still have lots of fun using dildos or just having alot of oral sex. But it is pretty rare that Two Tops pair up for long. They are both dominant in the bedroom and can't satisfy eachothers needs the way two bottoms can adjust to.
Ideally of course the pair is a top/bottom or at least one of them is Versatile.
And some guys over time change their preference. But these guys were most likely Versatile to begin with.