maybe it's both: christians look down on premarital sex, people look down on the homeless. take a christian, and he'll do both. thus, the root cause is prejudice against those who are different than you.
as for why... well, it's easier to hate irrationnally than to understand. it doesn't challenge you as much.
concerning why people think premarital sex is evil, i'd say blame abrahamic religions for that one. check out pagan, deist, and non-abrahamic religious rituals and you'll find a lot of sex in there. besides bacchus, i know of the vestals, some hindu rituals, amazonian fertility dances, hieros gamos, and even the pharaohs had to whack off into the nile ceremonially to guarantee a bountiful harvest (pharaoh = living god, giving his seed into the life-giving nile). sex is demonized only in western religions since the advent of christianity. before that, jews weren't numerous or powerful enough to have an impact.
also, not talking about roman orgies. there was very little religiosity about those, just massive parties where excess and hedonism were the only goal.