Well, not really.
Say if another Pokemon has a bunch of forms - they don't mean different genders, really. I can't say anymore on the subject, really... but interestingly:
Boys have the X and Y chromosome, while girls only have a X chromosomes. That negates this case, as the Mega Mewtwo Y form is being accused of the female form, while boys are the only one of the sexes to have a Y chromosome.
Nothing that's really to prove this case much, but I guess it helps me. xD
My point is - the Megas don't have gender, so basically their gender is completely unassumable.
Sometimes there are Pokemon with some weird genders - Female Machamp, anyone?
Hope I helped. :)