But then the section(s) of society you're trying to be fair to don't seem to have a level playing field themselves. So preserving the status quo isn't exactly sensible either.
AOG: Perhaps. The broader point is that Parliament shouldn't be made up of a larger proportion of black people/ women/ trans* people, etc, because only these people can understand issues specific to them (although in matters of politics personal experience is a massive advantage) -- but because if political interest is uncorrelated to ethnic background or gender then it follows that the make-up of Parliament should naturally reflect these diverse backgrounds in roughly equal proportion to the make-up of the total population. It's a simple statistical argument:
1. There should be no correlation between a person's gender/ ethnic background and their interest in a career in politics, or their personal ability to pursue such a career.
2. There should be no discriminatory barriers that would adversely affect a person's political prospects based on their gender/ ethnicity/ etc.
3. Therefore, Parliament should be approximately as diverse as the population.
If it is not -- certainly to the extent of a 4:1 split in favour of men, for example, it's clearly not -- then one or both of the first two points above is evidently false. I expect that it's going to end up being point 2, since this covers rather a lot of "sub-points", from the way a woman/ black person is perceived by the selection panel right down to how their ethnicity or gender affects the way they're perceived or treated in the educational system.
One way or another, though, there are clearly problems in society today that affect the career prospects of certain groups of people. This is true in not just politics, of course. But there is still some way to go until we genuinely are picking "the best person for the job".