Winston writes in his diary about sex. Not that this has anything to do with the brunette.
He starts off by discussing his encounter with a prostitute in 1981, moves on to his fifteen month marriage to Katharine (whereabouts unknown for the last ten years), and finishes off with the Party's denouncement of physical attraction and sex for pleasure.
Winston hates on the unthinking, brainwashed followers of the Party, such as Katharine, as he continues to reminisce about sex with her, which she saw only as a "duty to the Party" to make baby comrades.
Winston just wants someone to love. Oh, and to break away from the chastity that is so deeply ingrained in Party loyalty. The sexual act, naturally and lewdly performed, is rebellion against the Party. Desire is thoughtcrime.