I strongly suspect that lesswrong.com has an ideological bias in favor of "morality."

It does.

There has been no proof that rationality requires morality. Yet I suspect that posts coming from a position of moral nihilism would not be welcomed.

In general they are not. But I find that a high quality clearly rational reply that doesn't adopt the politically correct morality will hover around 0 instead of (say) 8. You can then post a couple of quotes to get your karma fix if desired.