Many young guys don't want more than sex... it's their nature to desire variety. It's a primal thing, so blame Mother Nature.
The thing about a lot of men, i've discovered, is they don't put their cards on the table and tell women they are only looking for something casual, not a relationship.
Women tend to have more romance and commitment in mind.
These things re not necessarily true for all young males and females, but for the majority, i believe it's true.