In an interview he explained that to make sex scenes interesting at all in books it takes space and work to make them good enough to justify that space. In his historical novels he thought he needed them, in WOT he decided it wouldn't bring much to have sex scenes and he had most of them happen mostly off-screen, among other things because he needed that space to develop other more important aspects of the story. For the ones he had, he didn't lose space on erotica itself.

He then added that there's a lot of sex happening off-screen, that for instance Mat has had sex with most of the willing women he canoodled with during his travels, some older than his mother.