I have yet to see Birth, so I haven't seen the controversial scene in question gee, but I understand your point.
I think the film would be more controversial because it's not considered as provocative to show a topless boy as opposed to a topless girl (I'm assuming Cameron is shirtless in this scene). The climate being what it is in the US, it is questionable whether a scene such as this could be included in any film being released in this country. I think were the situation reversed as you described, this film would've had to have been made in Europe.

I think it was the European press that got in a tizzy  when Birth was shown at the Venice Film Festival.  The scene really doesn't amount to that much.  Remember Justin Henry meeting a nude JoBeth Williams in the hallway late one night in Kramer vs. Kramer?  That was much more graphic although Justin was in his undies.  I think they were both in the same frames.

Putting a  10-year-old girl in a tub with a grown man would be more acceptible in Europe than in the US although girls and boys that age don't look much different shirtless.  What was the name of that movie where Liam Neeson took a shower with a little girl?

Here's Cameron in the bath with Nicole:

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What seems more exploitive to me is not the bath but Cameron's strip that precedes it, somewhat reminiscent of David Elliott in The Possession of Joel Delaney.  However, it is less explicit in the end.

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