Going back to the OP, while it may be a stereotype that it's men who want sex and women who go off it, there is a lot of truth behind the stereotypes as borne out by statistics.
However, I think there is a further, more important and unexamined stereotype: that men will take the lead when it comes to sex. This unexamined stereotype holds that the man will chase the woman, he will initiate, he will dictate the play. This may not be a strong as it used to be, but it is a very strong tradition, and is reflected in all manner of traditional etiquette: e.g. the man holds the door, and leads in the dance. Very sadly, in the past it has also been used as a justification for sexual assault: something overwhelmingly committed by men.
On Mumsnet I still see discussions on sex that tend to assume that all the woman has to do is turn up and look pretty. There are also statistics in Durex surveys that show submission is a female fantasy but it isn't for men, suggesting once again that men take the lead. I recently read a book by Stephen Pinker that suggests the male brain is programmed to seek out sex, and that there is something predatorial about it that men need to control (and various moral strictures in the past have attempted with varying success to do this).
But if women do tend to be more passive in sex, it will get very dull for the man in a LTR. Medical reasons aside, people don't generally go of sex - they go off sex with particular people. A decade of sex with a woman who never initiates, never seduces and, when it happens lies there like a starfish, will be very unfulfilling. I believe this is more common than people want to accept, and if it happens, I imagine that after a few years the man will simply give up because it's just not worth the trouble and resort to one of affairs, sex workers, porn, work, computer games or gardening after a bit.
I think porn is a red herring, by the way. It is very often blamed on Mumsnet for ruining a couple's sex life. I would like to see some evidence for this. My view is that if a couple have a good sex life, porn is not going to interfere with it. How could it? Sex has many dimensions porn doesn't have. However, if a couple's sex life has petered out, porn is a very obvious alternative sexual outlet for a man, and is therefore a symptom of a bad sex life rather than a cause.
It would be great to see some more research into male sexuality, and hopefully this is going to happen some time soon. Female sexuality has been the subject of a great deal of research in recent years, as borne out by the cornucopia of scientific research papers and sex toys. Men's by contrast, has been somewhat neglected, probably because men aren't so interested in male medical issues as women are in theirs.