Are you talking about visual arts, or erotic writing? By 'drawing the line' do you refer to societal, market-driven, or self-imposed limits on sexual content?
Everybody's kinks are different. I've done some heavy research into de Sade and Masoch, for non-kink purposes, so I don't tend to find them arousing at all. (Cringe-worthy, sometimes, and even laughable.)
As an artist as well as a writer, I find I'm attracted to visual works that show artistic merit as well as hard-core details. And in some cases, less detail is more: an image of someone's face *in extremis* can be much more glorious than an image focusing on just penetration.
If you plan to do art for a specific erotic market, first read and understand their guidelines and limits.
Here's a great site to see how other artists have addressed the genre over the centuries:
http://eroticarta2z.com
Good luck!