For decades there was a Comics Code Authority that put its seal on comics, but starting in the late 70's the artists started balking, and there really aren't a lot of specific rules besides what the companies think the readers will stand for. Generally a heavy PG-13 is the worst you'll find in today's comics, although some companies have been pushing the envelope again.

I'm up for seeing a nude Dawn ... but not having sex with Andrew. :-p