I have long enjoyed the sight of an open mouth during sex.
Not that I am a mouth fetishist or something. I don't particularly get off on the sight of a widened hole with a tongue and teeth in it. In fact, that's probably not something that turns me on at all. But what I like is the cause of opening one's mouth during the act of sexual intercourse (which, as the textbooks, will have you know, is a very important part of a loving relationship between a man and a woman).
It's usually an orgasmic moan, but there's other causes. Perhaps it's something more akin to surprise, or amazement, or maybe your muscles go slack during sex - in which case, consult your doctor. It's just that your mouth doesn't usually open that wide other times, unless you're watching a particularly good film.
An open mouth, like open legs - or an open heart if, like me, you are a romantic fool - is an unconscious sign that you are enjoying the sex. It almost gives your enjoyment away (if, for example, you couldn't hear, a mouth open with that wonderfully 'oh dear God that's good' expression would be a good indicator you're doing well). Shannan Leigh (pictured) is particularly good at conveying this during her sexytime scenes, but it's something I also noticed while having energetic sex with TD a couple of days ago.
So now you will, too. Another small factor in the wonderful world of sexual expressions.
My, I'm in a funny mood this afternoon.