Why does Winston call his and Julia's sexual encounter a political act?

Afterand's first sexual encounter in the
secluded meadow, Winston views the affair as a "blow struck against the Party" and
perceives their sexual encounter as a solely "political act." Winston has yet to
develop genuine feelings for Julia and his entire existence is impacted by the Party's
oppressive presence. All of his energy is focused on opposing the authoritarian regime, which he
realizes is a suicidal mission. Winston's desperate attempts to challenge the Party comes to
fruition by successfully sleeping with Julia. Winston is not so much concerned with satisfying
his physical desires as he is with disobeying and outwitting the Party. Essentially, Winston
takes more pleasure in committing a serious offense...