I agree that it's a valid question. There's no Scriptural evidence or requirement that He be of her egg. There is Scriptural support for her not being so. In short, if I had to choose, I'd say the egg wasn't hers.
Is this a standard Protestant belief?
"Belief"? I'd be shocked if it's the subject of any doctrinal statements either way. We all know she's His mother. Thus, the overwhelming majority likely assumes she's His biological mother without giving it any thought. But, search the Scriptures, and the question is (surprisingly, for some) totally validated.
It's completely understandable how some refuse to acknowledge that validity. It would ruin their whole day.
Lets see it.
BigMack