IMHO a lot of female writers (and let's face it, most of the writers are female) feel the urge to, I don't know, "feminize" one of the characters, to make one a "bottom" and the other a "top" while in RL most gay men are pretty versatile, and a very high percentage really likes anal sex. Sure, there are exceptions - but not very many.
For some reason Draco (in fic) seems the more feminine of the characrers. I've never gotten it either.