I can think of a few potential reasons: that the Pagan community is more welcoming of such interests and/or orientations than general society; that as part of examining one's life and one's self, Pagans are more inclined to examine their sexual/relationship life and be open to alternatives; that some common Pagan practices such as ritual nudity etc already move into the realm of what society may consider deviation from sexual norms, this making further explorations more acceptable to practitioners; correlations with other communities more keen to explore alternative sexualities, such as hippies and geeks (although this still doesn't adequately explain why);or that magic-workers are often interested in power and its relation to the self, and may choose the bedroom as a place to explicitly explore thisI answered:
I'll echo other's statements that most pagan 'religions' do not encourage or discourage sexual activity. You mentioned ritual nudity. The key word there is 'ritual'. When kept to ritual, the nudity does not affect the extra-curricular practices of the participants anymore than wearing revealing clothes at a fancy ball does. You also mentioned the power of sexuality. This is not exclusively a pagan secret. Secular advertising knows about the power of sexuality. The people at the beaches and nightclubs, dressed in oh-so-little know about the power of their sexuality. Even the Christians do, embracing adventurous sexuality as a way to cement a marriage.
So what's the difference? Paganism comes with less rules about sexuality. The contrasting Abrahamic religions have LOTS of rules regarding sexuality. The natural sexual tendencies of humans - straight, gay and bisexual - vanilla, S&M, BDSM - are not roundly condemned by paganism. In theory, they are not part of "paganism". Instead they are merely part of being human.
Regarding promiscuity, the desire to have a monogamous mate is present in the pagan world. Good sexual hygiene and focused emotional lives are valued by pagans. These are not inclusive to monogamy, but the exclusive relationship or extended-monogamous relationship does help these factors. While there is a greater spread of alternatives to monogamy in the pagan world, they do not lessen the sacredness of monogamous relationships that do exist. I speak from experience here. I am thoroughly pagan AND in a monogamous relationship. When that choice is made, the paganism and monogamy are very compatible.
How about those "deviant" practices? Are they "more" pagan. I would say not necesarrily. I have pagan friends who roundly hold this view, finding the view that "S&M" is pagan trait to be abhorrent. The deal is that they personally don't approve of S&M, and thus, don't want it forced into their world with the view "you have to accept it because it is pagan!!!".
I strongly suspect that one would find equal amounts of "deviation" and "kinkiness" among all religious groups. Some religious groups do condemn these acts and the people are driven into closets. I cast doubt on the assertion that pagans are "more sexual". Instead, I think they are merely more comfortable with their inherent, Goddess given sexuality. Kudos for pagans. Everyone else will have to stay in their closets!