Guys will be guys.. ones sex drive you might say goes on a scale of 1 to 10. Those on the lower end can be more comfortable around nudity,physical contact,porn and so on.Those on the upper end find themselves more aroused.
For women there is more emotion with sex. With guys its more physical. (Daa!)
Your good "friend" is not a good friend if he is trying to coarse you into something you don't want to be part of.In a strange way,to satisfy his emotional needs, he finds comfort with you and sex is the icing on the cake. These needs I believe is why so many women give into sex.
Help your friend by suggesting he go for therapy.Help yourself by doing the same.