If it were only so simple. Sure, simple statistics might explain it, but it doesn't really satisfy the question of which way causation flows: are foxes seen as sluts simply because of frequent occurence in erotica, or might it be that people who desire to be frequently seen in erotica have a tendency to be foxes?
(Good god, I slaughtered grammar there. I'm trying to say that cancer might cause mobile phones, if you catch my drift.)
Some people associate foxes attractive Mack-daddies and hot-momas.
It doesn't help that the fandom portrays them as promiscuous and extremely submissive murrholes.