While the odds favor you, please don't make sweeping generalizations like that. There are things about this 20 yr old that make me believe he is socally backward and may really not be after sex. There are also actually guys who were raised properly and are gentleman who would not take advantage of a girl. Yes, they do exist.
Girls should be very cautious about an age gap like this, but its not a given that the guy is after sex from a younger, impressionable girl.