Here is something I posted on an orkut community on the topic of why sex evolved. I thought I would post it here as well:

One prime problem we have is our inherent nature to 'bucket' things into various categories. "This is species X, that is species Y"... "This is male, that is female", etc.,. while nature does not have a mind and hence does not bucket its produce into such categories. For instance, we use the term 'eunuch' to refer to any individual whose sex is not entirely male or female. My question is can you fit all eunuchs into one single bucket called "eunuchs" without making a gross simplification? Some eunuchs are more male than the others and so on. Isn't it? What I'm trying to drive at here is the fact that reality is finely graded. Only for our understanding do we end up calling anything as 'male' and 'female'.

Okay... I sorta digressed ... but only to explain why 'male' and 'female' are being perceived as mere buckets and the underlying fact is explained below:

Sex could have originated from the fact that if two simple uni-sex individuals could reproduce and if due to their genotypic differences they showed stronger ability to survive, so be it! Selection will favour genotypic differences. The more differences you have, the more better. "More male" and "More female" starts being favoured. Clearer "roles" for each kind of sex evolves. Its like Sympatric speciation except its not geographical boundaries that helps speciate but sexual selection itself speciates them into 'males' and 'females'. Why are some women "very feminine" than the others? Isn't this a question that needs to be pondered over?

I think all sexually reproducing things have the inherent ability to 'detect' "femininity" and "masculinity" simply because of this underlying 'strategy' that selection favoured: "How good a *complement of me* is that individual to help me produce a robust off-spring?"