In many cultures, including large parts of the U.S., becoming sexually active is essentially a Rite of Manhood. For a young male in his teens, as opposed to one who is prepubescent, the thought of exploring his sexuality with an experienced and attractive older partner is entirely natural, as it would be for a female did her social upbringing allow it. Historically people tended to marry and start having kids in their mid teens, so that's hardly sick, just because we have arbitrarily raised the starting age to conform to certain peoples concept of what their kids should do doesn't change the biological drives.
You should also bear in mind that the statutory rape laws were not enacted to protect children but to protect fathers whose property, the daughter, might be devalued on the marriage market. Marrying a virgin maximized the chances of a mans heirs being his own kids, and that was a key issue. If a woman was sexually active the matter was in doubt. That didn't apply to boys, so the rules were different. The idea of applying stat rape laws to older women and teen aged boys is fairly new, going back no further than the '60s, and people not raised with such a viewpoint are likely to consider it ridiculous. Rape is understood to involve an unwilling participant, not a willing one whose parents disapprove.