I was prepared to write a novel of a response to this topic, but since the moment has gone, I'll just say this one thing: no matter what either side feels about this, people will like what they're going to like. You guys and gals can spend all week debating this, but it won't change the fact that there'll be some who prefer to write with the opposing gender. Whether that's because they feel more comfortable with the opposing sex as their writing partners or it has sexual/romantic motivations behind it, no amount of debating and/or bitching will change their minds. 

And whether or not you are willing to accept this is entirely on you. The fact of the matter is that people are stubborn. They'll do whatever they want regardless if others find it sexist and/or discriminatory.