(I'm usually happy for the gratuitous sex scene, I must say. I don't understand readers who skim the sex scenes. I should, and I don't condemn them, but I just don't understand it.)

Because I find most sex scenes about as interesting as reading about/watching someone using the toilet to relieve themselves. I really don't want a detailed description of a character's bowel movements even if it is part of the biological process, happens regularly (we hope), and provides a physical release.

Hope that helps in understanding.