In Sheldon's mind they are already there. His being in her apartment after dark, a kiss which he found fascinating, having to win her back from Stuart (and wanting to do so), asking to officially be a couple, a relationship agreement, etc.
He and Amy in his mind is very similar to Penny/Leonard/Howard/Bernadette (Raj left out intentionally) meeting someone, dating a few times, perhaps even sleeping with them, etc. For Sheldon, it's a similar commitment and personal emotional investment at this point already.
It takes more for Sheldon to craft a relationship agreement than it does for Penny to remove her clothes and have at it with someone. Sheldon is incapable of that type of emotional investment anyway.