Why is Winston happy to hear about Julia's alleged promiscuity?

Yes, isn't that romantic? For Winston, Julia's promiscuity means primal passion and animal instinct. Winston thinks uncontrollable desires like this weaken and can destroy the party. Pent up sexual tension can only be channelled into hate for so long. Winton hopes that Julia has slept with hundreds of me: this means that party members aren't completely submissive and living in fear.