I do think exclusively writing one or the other, at all times, is a sign of weakness in a writer.

*nods* I have written at least one story that was 98% one gender (female), and it was one of the easier fics I've written. While I appreciate that, I don't see it as a sign of a good thing. In fact, I've seen an all-female longfic that practically screamed "Can't look outside of self when writing characters."