There is a very straightforward reason why there are fewer male teachers and it is a very feminist issue.
If you look over history of professions in last 500 years you will see that the level of respect of individual professions relative to others matches the rise in female representation within these professions. So, judges for example are still seen as height of intellectual achievement, architects are highly thought of and so on. However, where teachers used to be considered pillars of the community 100 years ago, that respect has dropped away and the profession is not held in such high regard as it used to be (although of course, it is still a profession and ranks accordingly, I am just speaking in relative terms). Same is happening with GP profession where level of respect is nowhere near the levels of 100 years ago (although decline is slower because we have only had very high levels of female representation for 20 years or so). Legal profession, too. Interestingly you can see that even within the professions themselves, there is distinction between respect accorded headmasters, professors and surgeons (mostly male) and that accorded lower ranks (mostly female or mixed), and a further sub-division between strands, such that science teachers for example accorded more esteem than arts teachers. Again the gender split matches the level of respect.
Why? Research seems to imply that society as a whole buckets work into 'male-dominated', and 'female-dominated', and, as a profession switches the gender balance, so society values it differently, and also rewards it differently. This value and reward thing then sets a cycle in motion that creates an impression of gender stereotyping and low levels of pay. There is a strong correlation between respect accorded people and the money they earn, and this works against teachers and against attracting wide ranges into the profession.
That decline in pay can be explained by a simple economic model. Say there are 60 women and 60 men in a society with 120 jobs - 40 teaching jobs, 40 mechanics jobs and 40 fire fighter jobs. In our generation we now tell out children they can do any of these, but this was certainly not the UK situation immediately after WW2 when women wanted to work in very large numbers for the first time. So, back to model. 60 women look at the jobs on offer and the car/fire jobs simply aren't open to them, so they try to get the teacher jobs whilst the 60 men look around at all three jobs. This means appx 80 people (60 women and 20 men) chasing 40 teaching jobs, and 40 men chasing 80 alternative jobs. Simple supply and demand models dictate that salary of teachers will drop in this model until a level where half the applicants walk away, and salary of mechanics and fire fighters will rise until they can attract enough candidates. So, society creates the pay structure and inbalance that then labels a profession and puts off large sectors of society. You can see it in other professions too, working the other way round (Chemical Engineering as a career choice, anyone?)
There's always hope that in the next generation or two we might actually make headway into kicking all of this stereotyping into the long grass (although the lack of female role models makes me despair). In the meantime some of us should ask ourselves why we don't respect the primary school teacher as much as we respect the local vicar, doctor and judge. Because 100 years ago they'd all have been pretty much in the same bucket of respect, and nothing fundamentally has changed other than proportional representation. We are obviously our own worst enemies.