What does the physical act of sex symbolize in chapter 2, part 2
Seeing as no one else has answered this, I'll take a shot, but I'm a little unsure of what you mean. Do you mean that it actually is symbolic of something other than sex? Or are you asking what meaning it has outside of just being a sexual encounter?
The second of these is much easier to answer. What Winston and Julia are doing is much more than just satisfying their sexual urges. Winston, especially, sees the sex as a blow against the Party and the society it has created. That is why he is so excited by the idea that Julia has had sex with Party members and by the idea that she likes sex for itself. Those are both really subversive -- they would both help destroy the society the Party has created.
As far as actually symbolizing something else, I guess it might symbolize an act of aggression against the Party (but Winston isn't particularly aggressive in this sexual encounter). It also is an act of freedom, just as the girl's taking off her sash and clothes (in his dream) were an act of freedom. But beyond that, I don't see how this symbolizes something else. To me, they are just having sex, but the sex is, for Winston, about striking out at the system more than about physical pleasure.