Male orc characters' population outnumber females 9 to 1, the Alliance just needs to sit back and wait until they kill themselves in a couple generations.

Of course, they may not be alone in that. Dwarven and Taurens are also endangered but I can see them holding back for a couple of years so they can re-fill their ranks. Orcs will just go "For the hooooorde" and charge their way into extinction.

Meanwhile, a young orc may not even know what a female orc is, hopefully they will not develop some weird fetishes and end up on the Blood Elf camp. These girls are gross!