Well, you'll get your chance when the source is released at version 1.0.0, hence the saying of "maybe next time".
But really, DoomGuy is DoomGuy. Gender equality aside, the amount of effort required (i.e. all those itty-bitty pieces of code to check for may/femay to output the correct pronouns) is a bit too high at this point. Playing as male or female shouldn't affect one's ability to destroy all those pesky hellspawn.
And why is it too high an effort? Because the main code base is still in a flux, and I mean this from gameplay and cosmetic perspectives -- adding all these little checks is very error prone and under the circumstances, should probably be minimised while we "finalise" the gameplay for the Great One Point Zero release.
Trust me. Once we get 1.0.0 out of the door, we can add all the niceties like proper plurals, translation, gender-confusedness and what-not. But for now, please bear with our [unintentional] chauvinism in stating that DoomGuy is a male.