For starters, the issue isn't remotely as rampant for men as it is for women who routinely (i.e. multiple times a day) receive unsolicited sexual advances offline and online in the form of dick pics. It's also much easier for a man to physically take advantage of a woman than vice versa, so women would naturally feel much more threatened by this behavior. Men's inboxes just aren't constantly getting filled with unsolicited nudes, and those that are received rarely cause men to feel concerned that it could lead to real aggravated sexual assault.