Double Rape... and Aaron?

"The emperor's court is like the house of Fame,
The palace full of tongues, of eyes and ears:
The woods are ruthless, dreadful, deaf, and dull.
There speak and strike, brace boys, and take your turns;
There serve your lust shadowed from heaven's eye,
And revel in Lavinia's treasury."
(II.ii.126-131)

It appears that Lavinia has been raped twice in this play already -- figuratively in the first act and literally in the second act. Is there a reason for so much emphasis on rape/sex (eg, Tamora getting intimate with Aaron in scene 3 as well), or is it just there for drama/action? I will admit, it was fairly clever of Demetrius and Chiron to not only cut her tongue out, but also to cut her hands off so that she could not reveal them.

Also, it is interesting how the significant characters of a play are introduced in some way usually in the first act of a play. However, in this play, Aaron had no part in the first act (except entering the scene as "The Moor" with Tamora and her sons), yet his role is significant in the second act as it is the influence and motivation behind most, if not all, of the action that occurs. I will admit that it caught me a bit off-guard. One would have expected the quarrel to continue with the two who had the greatest motivation, either Titus or Tamora, but instead, it is driven by this character who we know very little about -- other than the fact that he is a Moor and Tamora's lover. What motivation does he have to seek revenge against Titus? Is it his love for Tamora? Is it the fact that he is rejoicing because Tamora is now in a position of power and he hopes to advance himself through her? Or is he simply a vengeful character who takes pleasure in committing evil acts without motivation?

Finally, as a side comment -- there is no doubt that this play is a huge mess, which I guess is also easily reflected by how confusing it is to follow.