There are men in SL playing women, but they are dead easy to spot. So if I, as a guy playing a male avatar can spot them, I'm sure women can spot them even more easily.
As to the real life females playing female avies, in SL no-one is old - everyone plays someone who is young and sexy. If you are somewhat less than attractive in RL, what's the point of playing an equally unattractive avie when you can go for uber sexy? And if you've got it, flaunt it!
As has been pointed out, there are many housewives looking for a bit of excitement in their life which they can't irl. They may or may not be a MILF in real life, and even if they are they probably wouldn't behave as they do in SL - but what's wrong with living a little fantasy?
SL gets an often unfair press - slagging off SL because of the amount of sex there is a bit like saying anyone who uses google is a saddo perve. Just try putting in the most harmless of google search terms and you can guarantee to have some pron links in there. The interwebs is awash with pron as is RL - SL merely reflects that, albeit in a somewhat more open fashion due to the anonymity of your avie.
All in all, it isn't a great surprise that female avies in SL show more "flesh" - just as it isn't much of a surprise to discover that "hotties" in real life show more flesh.