To the extent that Julia engages in lovemaking for an act outside of procreation and is well aware that this is against Party rules, there could be an act of resistance seen in her actions. Whether or not Julia sees sex as an act of political defiance is a bit murky. She is described as a "rebel from the waist down," which does reflect that she is seen as a political rebel that does believe sex as a physical expression of such resistance. At the same time, her preaching of Party values, but living in a contrary manner might not be political resistance as much as personal expediency. We have little indication that Julia actively embraces sex as an expression of resistance, but some notion that she is aware that she is going against the will of the party. In this act, while she might not see it fully as that, there are political implications to what she does.
I do not think that Julia really sees the affair she is having with Winston in a very political way. Instead, she sees it as a physical thing and, perhaps, as an act of personal (as opposed to political) rebellion.
When Julia and Winston start their affair in Part 2, Chapter 2, Winston is all excited by the fact that Julia has had sex with Party members. He loves that she has gone against the Party. But she says nothing political like that -- she just says she likes sex.
I think that she is rebelling somewhat, but it is just against the fact that her ability to have fun is being limited. It is not a rebellion against the political ideas of the Party the way Winston's rebellion is.