How common is rape?
Rape is the most common serious crime against women. Although the reported incidence varies somewhat from year to year, most authorities agree that the number of rapes far exceeds the numbers actually reported.
Should I try to talk him out of it?
You should use whatever method you think will be appropriate. For a rapist who is fairly casual about the situation, it might be possible to talk him out of it but, in most cases, that's not something you can count on. You should definitely have a repertoire of other things that you can do if talking doesn't work or if there isn't time to talk
How serious is rape?
Very serious. Serious enough to be legally classified right up there with murder and robbery as the most serious of crimes. Even if a minimum of force is used to carry out the rape, the possible consequences to the woman (pregnancy, AIDS, herpes, other venereal disease, and post-traumatic physical and psychological effects) are extreme, severe, and very long-lasting.
There are many different kinds of rapists, ranging from the easily-discouraged to the sociopath who has decided that he won't stop until you are dead, or he is.
A recent AWARE blog entry provides information about a 16-year study of rape in the US. It shows that, while the number of rapes has gone down a lot, more rapists are armed and more rape survivors are being injured.