The gun lobby is flirting with self-parody as it exploits the issue of sexual assaults on college campuses by proposing a solution of — what else? — having students carry guns. Experts who study the complicated issue of predatory behavior and advise colleges point out that rapes often begin in social situations. “It would be nearly impossible to run for a gun,” said John Foubert, the national president of One in Four, a rape-prevention organization.