Acceptance of a potential outcome is not the same as acceptance of an eventuality.
I can consent to being dropped out of a plane (go sky diving) while accepting that there is a risk that I will die, but that does not mean that I have consented to being killed. That does not give someone licence to push me out of the plane without my parachute for example.
If someone consents to having protected sex, while understanding that there is still a risk of pregnancy, the expected result of the encounter is that no pregnancy occurs. That is the result you have consented to, even if you recognize that there is a risk it will turn out differently.
To borrow your synonym. You have "given the thumbs up", "accepted", "approved", "given permission" to having sex. It is the sex which you have given this affirmation to. Not pregnancy. Even if you recognize the risk thereof, you have not given approval for it to happen.
Even though "permission", and "authorization" are both synonyms of consent, I'm sure you can recognize that you cannot substitute those words into all of the same places. There is nuanced difference to their meaning.
You'll notice that the definition of synonym, "a word or phrase that means exactly or nearly the same as another word or phrase in the same language, for example shut is a synonym of close." does not require that the words share the exact same meaning. It can just be similar meaning.