LOL thir. Naked on your knees is a bit much but getting slapped (meaning on the face) is a bit softer? weg. Slapping a woman for the first time for her sexual pleasure was the most difficult thing I ever did in the lifestyle. Talk about overcoming a social taboo. Spanking, especially otk, was not merely easy in comparison, but definitely hot, even for most vanilla men.
And comic books, hollywood media, and even mainstream art, all depict bound women as somewhat normal. Police dramas, detective stories, and even the occassional sexual "misadventure" have inured men to enjoy it visually. The only "issue" is what will he do with an actual invitation to partake.