If you want to make sure that someone your with intimately consents what legal document should they sign?

I know this may not be he answer  you're looking for, but if a sexual partner later claims that he/she did not consent to sex, either because he/sh was forced, or was incapable of consenting due to age, lack of mental capacity, intoxication, etc, don't you think that person will make the same claim about any document he/she signs purporting to give consent?  If the person is too yound or drunk to consent to sex, or if they are forced to have sex, signing a document while underage, drunk, or being forced to sign won't change anything.