Raam, the reasoning I have about Ravana was not after Sita are:

1. Sita was an orphan. There is a story that says she was born to Ravana but due to astrological evidences,ravana had to part with her as a baby.(Late R S Manohar's drama)
2. He did come to the swayamvaram,had he wanted, he could have abducted her then. He did not.
3. Had he still wanted her, waging a war against Ayodhya would have been an easy victory. He did not.
4. Rama had trespassed into Ravana's land.
5. Sita did not heed to her instructions.So she could not have done anything at Ravana's place either.
6. His rule was peaceful, hanuman himself says this. For a ruler of that nature, the thought of marrying a child(when compared to him) would be a big no no.
7. assuming sita was Ravana's daughter, there are enough human nature in Ravana to bring his daughter home. MandOdhari and soorpanaka might have been against this and hence the ashoka vanam for sita.

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