I think there are a few different olfactory issues at play in this thread. Let's start off with 'the smell of sex,' - to me this is the combined uhhh, musk of a guy and girl throwing down and both sweating due to exertion, which is combined with the scent of a woman's junk going into 'get slippery for sex' mode. That's different than the normal smell of a guy (oftentimes slightly sweaty maybe mixed with the lingering smell of soap and/or deodorant). Likewise, I think women have a normal scent quite different than then what they exude during sex.
Not trying to be crude here, but in my experience every woman's vag has it's own unique scent-based calling card. This has a lot to do with her age, diet, how much water she drinks, if she smokes, and I guess how much beneficial bacteria reside in her system (eat your yogurt ladies!)
In the act of getting aroused, women's junk obviously start producing fluids to aid in making a wang do his job. In my experience, the scent involved at that stage, down there, has a common element to it that I've noticed with pretty much every female I've ever had the pleasure of being with. I think it's this common smell that women can sometimes recognize, and if they know they didn't make it, well cue the accusations!

But that's different than a girl who has sex while she's got "a cold." An upset naynay getting snaked can indeed leave a funk in the air. Sorry ladies, not trying to offend!

Women tend to pay more attention to the minutia of body language then men, particularly the eyes, so I feel it's pertinent to mention that sometimes the 'I smell sex' angle is really just an excuse for getting other hints of guilt.

my $0.02...