It's an easy question. Yes, sex does sell. Some people find it vulgare and disgusting and would say that it's wrong and doesn't sell, but in truth, it does sell.
Classy is usually connected with being sensual and sensual with sexy, just look at this add by Smirnoff.
[link]
And I bet your mind travelled through that very line of thought when you look at that picture.

One thing to be noted is that a lot of sexy adds are aimed at men, in the way that it usually has something to do with sexy women.(How that might be directed at females is that females might think 'Hmmm, would I look that sexy if I drank that?' )
But here's an Absolut Vodka add that is definetly aimed that females. (any really sensetive people should not look.)
[link]

Yeah, sex sells, but I would really want to add a 'Y' at the end of that and make it 'Sexy Sells', which I think would be more pin-pointing what it is.
To sell something it doesn't have to have sex in it, heck, doesn't even have to have a person in it, as long as it's something sexy that makes you think "Sex".

I'm not sure if many have noted this, but if you look as sports cars you'll find that they often are designed to look sleek and sexy. There's another thing where sex sells. Just because something doesn't scream "SEX!!" in your face doesn't mean that it doesn't add to a persons general sexiness if they have it as an accessory.

Porn is very straight forward, personally don't like it much cause to some degree the fakeness of it makes me sick in my stomach.
But as for sex or sexual things envolved in comics and stuff, there's nothing wrong with that. Dragonball is suitable for kids and have many different points where a characters privates are visible, but it doesn't matter because that it's the point of it, the joke that is made about it is.

Something can be more or less offensive (in a sexual way) depending on how focused it is on the actual 'sexual act'. Like is I drew a picture that was about one of my characters nearly dropping her towel and some of her privates being visible, the privates wouldn't be the main part, the point that it's funny that she's nearly dropping her towel is. Thus the effect of the 'sexiness' wouldn't be as brutal as in some cases where the nakedness is the main point of the add, picture or thing.

I feel I've started to ramble here, so I think I'll stop here, for now. :3