I think you missed the point of that sex scene. You are meant to contrast everything that happened in the latter part of the film with everything that happened in the begining of the film. A good deal of the movie is about how the "American Way" and the American Dream" is a false pretense that we carry to kid ourselves out of the fact that we a country of violence. From manifest destiny to Hiroshima to our current War on Terror, we solve our problems through violent outright agression. Though we cover it up with our ideas of morality (Tom wears a cross) and dreams of the white picket fence, when that dream is threatened we revert right back to what we really are, and it is this "history" of violence that is passed down to our next generation (really watch the scene where his son shoots Ed Harris and watch his reactions) despite our attempts to the contrary. Thus, the american fantasy of the cheerleader/homecoming queen getting it on with the quaterback in the first act is contrasted with what is almost rape in the third act. The smart kid beating the bully through his intellect and humor is contrasted with him breaking the kids nose later on. And in the end, no matter how he tries to avoid it, Tom/Joey has to inherit his house, and thus his history: he has to own up to what he is and what he has done. However, his family still pretends that their idyllic dream is the life that their living, even though all of them know it's a sham. And who looks up at the end? Joey? Tom? I say it is Joey, and that look of utter despair is because he knows he can never be Tom, that maybe he never was. What does that say about us?
You call that silly?