interactive sex is silly, i agree, but is there a way to bring depth and interaction to a relationship culminating in sexual encounter? i think the red engine will be perfect demonstrator of this possibility.
see, what happens is, after the initial stages of witcher 3 mania ends, and te devs release more detailed and specific videos detailing game mechanics, it may be possible that they will include a sex and relationships video showing how their branching storyline system can apply to relationships.
so for example, the dialog options you choose in relation to a potential partner will change their perception of your sexual prowess, measurement, etc, and you will affect geralt in how he perceives the potential partner, as either desirable, easy, irritating, etc. all will be indicated through dialog of course, or expressions, body language, etc.
now, there will be a few "sex states" for the relationships, so that when you do engage in it, the cutscene will show exactly how your choices have influenced geralt's actions. this can be done any number of ways, animations, cinematography, dialog, sound effects, etc. maybe the partner ended up annoying geralt, so now the cutscene begins with geralt play-raping her. or if you came upon an aggressive partner and did not impress her enough, the cutscene could simply show geralt curled up in the corner and the aggressive partner walking out of the bedroom with a glistening thread spool.
if your dialog choices reflected a timid approach, the sex scene may be subdued and the body parts will be covered by blankets. and if your dialog choices reflected an aggresive approach, there may be penetration.