Non-escorting women don't like it as they see it as dirty, degrading and possibly an easy way to make money. They don't really understand the job of an escort.
Men view it as taboo because women don't like it.
Yeah, I think this is pretty spot on.
Old fashioned societal attitudes towards sex before marriage and unplanned pregnancy have fallen by the wayside, so those can't explain why sex work is still taboo.
Ultimately I think the societal shame comes from women who are unhappy that the "rules" of sexual relationships are being undermined. Consider how hard a young guy may have to work to impress a girl on a night out, get her number, wine her and dine her, perhaps commit to a serious and monogamous relationship, all before her knickers come off. With a prostitute, you pay her money and her knickers hit the floor in the first five minutes. Then of course there's the competition anxiety from frumpy married women who don't really like sex, but hate the idea that their husbands could easily pay for sex any time they leave the house.
Likewise I think it circumvents the whole "game" of male sexual competition and is seen as cheating to an extent. At its core, the idea that men are the ones who pursue women, women are expected to resist, and only "give up the goods" after sufficient persuasion, is fairly sexist and should be opposed by feminists more than anyone - but it's a societal holdover that benefits women, so it's allowed to continue mostly unscrutinised. A man who declines to play the game, jump through the hoops and perform the traditional courtship rituals, and just skips straight to the finish line - he obviously gets some envious looks from all the blokes who are still grafting in the traditional way.
I think a lot of heartache (and ballache) would be avoided if both men and women were more honest about sexual and romantic needs, and the difference between the two.