The bases change dramatically?
Could you explain what you mean by that?
Most systems of sexual meaning are dependent on the economic organisation of "the family" as that low level economic instrument of production and reproduction. The meaning of gentlemen dressing as ladies in the 18th century is somewhat different to the meaning of men dressing as women in the 21st century; the very meaning of what it is to be a man or a woman in one and another setting is different. If the basis of masculinity in, for example, Australia in the 1950s involved full-time work, mate ship, and the attempt to produce an idealised family structured around a wife conducting domestic reproduction in a commodity market; this is a very different basis of masculinity to a frame weaver in 1780 whose entire family works an 18 hour day in the house at their own pace and leisure. The system of meaning is also a system of material culture.
If you mean to say that modern sexual practices as described in factual terms are not present in the past, that's a howlingly unfounded academic conceit.
Show me full body latex suits in the 12th century and I'll accept your criticism. Some of the practices of sexuality are the enculturation of material practices, including here affective practices as a "materialised" or concretised practice. Now human spontaneity might discover ways of doing what to whom; but whether these newly discovered material practices become culturally supported, widely spread, recommended or condemned is another matter.
The long debate in the history of sexualities over the role of male-male anal penetration in sex between men is illustrative. Isolated cultures of sexual conduct between men in the 19th century have preferenced frottage or oral sex. It isn't just the bells and whistles, or the names and handshakes, the who does what to whom in the where is highly context dependent. The acts themselves have meanings written into them.
That's what I mean by different bases. Of use for our speculation: given the construction of the courtly family as having an heir and several spares; the spares restricted from reproduction through late marriage, lack of land or church service; the desire for people to express their sexuality could be removed from the private sphere of "monks and nuns engaged in evil private acts" to a public conception of necessary display of devotion or affection on a courtly model as a mandatory act for non-married elite males.
Of course they are. The fluff and babble around them however, is significantly different, and they are highly context dependent.
I think I've covered this. What two or more people discover in bed by themselves isn't historicisable. What two or more people discover, and then produce a culture imbuing it with a specific meaning, that's historicisable. It is the meaning of acts that is the history of sexuality.
So for that reason I'm not seeing D/S appearing in exactly the same form as today in the middle ages. Certainly "slave" wouldn't have the dirty connotations that it has now as lots and lots of people were in servile positions, while "dog" would be pretty unspeakably vile as an image and probably not sexual.
Given widespread servitude as common, and the need of the medieval elite to jealously preserve public station (sumptuary laws, for example), I'd suggest that slave might be even more shocking if applied in the context of courtly love?
yours,
Sam R.