I know that's what they say, but I think they are speaking loosely. They mean that the consent is not properly informed and a "yes" has no weight.

Interpreting other people's motives for them again?

You need to break those bad habits of yours...

It's the inability to determine those circumstances in advance that make it always unethical to do it.

Yet you continue to refuze to sign for a ban on children being allowed in swimming pools and cars.

I may believe you when you stop being such a hypocrite.