It takes two to create the baby, they're both aware that getting pregnant is a possible outcome. And so they're both responsible for the baby.
And I think what you're forgetting in your 'group project' example is that having the baby is just one step to a giant 'group project'. You can't expect everyone to do everything in one huge group project, it's shared between the people. If only one person does the introduction paragraph and no one else, that doesn't mean no one else is going to contribute with what's further to come in this project?
So with that comment, you're backing up my point.






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