Over the years he has made occasional jokes or asides about things like spanking and crawling as therapy - those are just the first two that popped into my mind.

Also, just my opinion, but he embraces counterculture via the "goth dad" persona and that may contribute to the fan perception of him being more open to things like kink, whether that is a fair conclusion to draw or not. He also seems less easily offended or made uncomfortable about sex talk in comparison to some other people in the cast.