because in traditional cultures organised along hierarchic patriarchal lines, but realy even the few matriarchal ones we know, heterosexual intercourse and relationships automaticaly imply a multitude of complex relations of status, power, controll, group belonging, ownership, personal right, mutual cooperation or conflict, possible insults and injuries and debts and scores, duties, loyalties and responsabilities, all expressed trough customs, laws and orders of how things are done by whom to whom, when and and why
its still this way in gypsy culture for example, female family members are the real and most prised capital a family or klan disposes of, everithing else is just objectified value of some kind, but females are true wealth, all long term significant intra-relations withing the population itself are based on interactions revolving around arranged marriage and prodigious consumations of same, and one of the greatest powers the big baja has is that he can decide or veto such arrangements
obviously this leads to people being highly protective of their females vaginas, and not once was it known to occur that relating indiscreetly with a girl, even if she herself might be a bit slutty, got a man in serious shit when her 7 brthers randomly pop up from the first trashcan with cold weaponry, but still, this was relatively rare, as most gypsy girls heed this logic and do their best to keep their legs cosed if not othervise instructed by the appropriate authority, and, frankly, few outside people would ever even want to put their dick in them either, but you get the idea
so realy in a lot of cultures, especialy and specificaly the hard-core macho ones where females are just house appliances and baby machines, fucking a boy is just way way less trouble, and way way less likely to cause your dick being cut off by some angry dad and his 22 combined sons and sons-in-law, or, sometimes worse depending on who the dad is, making you the 23d
