Can it be moral for a mom to have sex with her adult son if they consent to do it?

In some cases it would be okay. For example, if a gang of home invaders held a gun to the mom's head and forced them to do it. Or if they held the gun on their dog and said "do it or we will shoot your Chihuahua". Then of course they would save the dog and have sex right there in front of them. Also it would be heroic. Another good and moral reason would be if the mom had to get pregnant to use the new babies after birth to help her other kid get well from cancer or something and her adult son was the only match. But he couldn't give his DNA or blood because he didn't have enough to spare. Stuff like for emergency reasons it is morally fine for sex between mom and grown up son. But not for any other lame excuses or if they are horny then no it is very sinful.

No, it's immoral in every society that I am aware of.

Like someone else said, if they were forced to do it by someone under a death threat, it would not be, since otherwise they would be putting the other's life in danger. But to do it for pleasure is immoral.

Not sure if I would call it moral per se, or particularly wise, but I don't see a crime or offense against the state or population.

It's against my morality, but I'm not the one doing it

In atheism maybe

I don't see what's morally wrong with it according to an atheist... especially homosexual incest

no, if the mom got pregnant, the baby could have serious deformaties for life and thats not fair to do that to an innocent baby

In what world is that ever fine?

No way

Crack isn't moral.

No