If there is the option for them both to be 'father', or 'parent', I agree it's very odd and off for one to choose to be 'mother' - but if there is no other option, what are they supposed to do? Forgive me if I'm wrong, but they don't know who the biological father is, do they? So they couldn't even do that (or am I thinking of another celeb couple?).
I agree with Lurcio that best thing would be a two part birth cert, so the child can find their sperm/egg donor when they are of age if they want to, but the social birth certificate which lists the parents who are raising them, and that's the public one :)