I have not really followed this in too much detail, but I was under the impression at first that it was the defence that brought up all the sex stuff because they were trying to paint as her a victim of sexual abuse. One the the deatails mentioned was that he had tied her to a desk and sodomized her. But if they had this whole s&m thing going and it was consentual, then yeah non of that is even relevant. What two consenting adults do behind closed doors is their buisness, so long as no children or animals are brought into the mix.
But I totally agree with you, none of this should have ever been brought up it was simply just to use as character assassination. What it comes down to was she being abuse and killed in self defence, or was she jealous and killed out out of anger.