When Winston asks for Julia to be put into room 101 in his place,how does that symbolize Winston's acceptance of Big Brother and the Party's ethos?

This is Winston's final betrayal of Julia. They had sworn to each other that the Party could never make them stop loving each other. Putting Julia in his place in front of the rats meant he had forsaken her in his heart. Once Winston has given up Julia, in his heart, he has effectively relinquished all control to the party. There is no love other than the love of the party, there is no reality other than the reality of the party.