To me, what I feel like they take away is valuable screentime. This opnion does hinge upon the fact that I feel most sex scenes are superfluous and don't add anything to the movie they are in. To be honest, these seems to be further backed by the fact quite a few sex scenes are done at the behest of sudio executives and not directors, because they know sex sells, at least with older generations. I feel like a lot of younger people recognize this and don't really care for a sex scene for the sake of itself.

This is all not to say I think there shouldn't be sex movies. I do think it can be part of the plot and add to the narrative, I just don't think that is always why we get one.