He told Nerd Cubed:
"The problem with this whole [female Doctor] argument is everybody [just] says 'a woman' - you don't ever cast 'a man' as the Doctor, you cast a particular man.
"One day I'm reasonably confident somebody in this [showrunner] role - whether it's me or someone else - will say not, 'Let's cast a woman', they'll say, 'That person, that's the one who could the be the Doctor'."Moffat explained that the casting process was not about "casting a gender", saying:
"You're just casting someone that you think will set that role on fire. I think for now that is Peter - I thought it was Matt [Smith] before - it's not about the gender thing."[source]