The following article from Alice Collett and Bhikkhu Anālayo was already mentioned once by @Christopher but I think it deserves to have a post of it’s own.
The terms bhikkhave and bhikkhu, and particularly their appearance in Pali Buddhist literature appear, on the surface, to be terminology that excludes women. The vocative address to monks (bhikkhave and its equivalents) that occurs so often in sutta literature appears to be indicating that the teachings being proffered are addressed exclusively to male monastics. Similarly, the use of the normative bhikkhu (and its equivalents), in expositions relating to the teaching, again appears to indicate that monks are the sole and only concern of those offering the teaching. However, in both cases, such an understanding of each term is problematic.
In this article, we discuss each of these terms, and look a little more closely at each, suggesting that in fact neither term should be considered to be exclusive language; that is to say, in neither case do the terms function as indicators that the address or the detail of the teaching is solely for monks. The term bhikkhave should be considered instead to be a form of—what we are calling—an idiomatic plural vocative; that is, a vocative that is intended to capture a broader audience than is implied by the actual term itself. Similarly, bhikkhu is intended as an umbrella nominative, to mean “monk or nun” and sometimes as well “laity” and should be read as generic. We first discuss the term bhikkhave, then bhikkhu, and following that we also include a note on the term arhanti.