2. Then hygiene. Especially if the girls are used to doing anal scenes, they keep that area spic and span back there. The dudes, not so much.
3. The third reason that comes to my mind would be that there can be a difference in attitude, if you will, behind a female-rimming-male scene and a female-rimming-female scene. This is by no means a hard and fast rule, and there are many exceptions, but often, a female-rimming-male scene has a tone of making the girl do something even dirtier than usual, whereas a female-rimming-female scene has a tone of pleasuring the other woman in another way. So the way the rimming is done is naturally quite different between these two types of scenes.
But this third reason does not always apply, so in those cases I would just refer you to reasons 1 and 2.
[Link]