With all this supernatural power....
you would think they could arrange for a male baby to be born somehow. Or at least force Julie to get pregnant from someone for at least a chance at a baby boy.
Well aside from the plot device of having a baby born naturally, I really think it was a 'free will' thing as well.
Like in "The Omen"'s opening, we see the evil priest tell Thorn that his baby died, and suggests that he 'substitute' the orphan baby Damien.
Why didn't they just switch the baby without him knowing? Why ask? Because it was a free will issue, it was part of the religious 'requirements'.
"I'd say this cloud is Cumulo Nimbus."
"Didn't he discover America?"
"Penfold, shush."