Hello fellow cozy writer! I finally found one!
I have to disagree with the "no rules" thing. When it comes to regular mystery books, that's probably the case. But there are definitely rules when it comes to cozy mysteries. I've been reading them all my life, I'm involved with cozy mystery groups online, I've met some cozy mystery authors and I'm friends with three well known cozy mystery authors.
What makes a cozy a cozy at all is the fact that they're light-hearted, not overly serious, lots of fun, and where justice is always served. A few authors have snuck some "minor" cuss words into their novels but, in general, cozy fans don't want it. Personally, if I run across more than one "d*mn," the book is done. If I see even one "b*tch," that author is done. Cozies don't cuss.
When it comes to violence, the murder never happens "live." Always off-screen. Cozy readers don't want to read/see the gruesome details.
And sex? Not in a cozy mystery. There are many romances and some of them might imply that a couple slept together but that's the limit. I can't think of any off-hand that even imply it but there could be. Sex is not romance. A cozy romance is about looking at each other, holding hands, sitting next to each other, heads on shoulders, hugs and a gentle kiss.
The three basic rules of a cozy are no sex, no violence, no cussing. That's what makes them cozy. They're books you can read to your grandma.
If you have a way to read books electronically, I recommend going to iTunes and getting Nancy J. Cohen's book called Writing the Cozy Mystery. It's only 42 pages (no fluff) and is only $.99. ($6.99 cover price.) You can read it in an hour at most. It's very helpful and informative.