Why so shy about sex in comics?

So many comics seem to be shy at showing hardcore sex.  Like in yaoi, they so often cover the sexy parts by strategically placing baloons or objects.  Why?  Are they afraid it becomes porn at a certain point?  It's just frustrating to me.

I like this webcomic - Kong

At least this guy doesn't shy away from the sexy parts.  Yeah, some of his stuff is just porn, but the Kong comic has a storyline to it at least... and he's 17 pages in before the sex starts... so IMO, not porn, but hot webcomic reading. 

Any other webcomics out there that blur the line between good reading and porn?