Why Ravan did not force himself upon Sita

Ravan forced himself upon many women. He molested many women (including Vedvati). Then why did he do no such thing with Sita? The answer given below is from Valmiki Ramayan, Uttar Kand.

Kuber was Ravan's step-brother. Kuber had a son named Nalakuber. He was married to Rambha (a nymph). One day Ravan molested Rambha in spite of her repeated pleadings not to do so. When Nalakuber came to know this, he cursed Ravan that Ravan's heads would burst if he did that again with any woman.

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I never thought about this

This is a very sensitive topic and I once read an essay debating what constitutes the purity of Sita. In that essay the author's POV is that the Sita's purity is in her thoughts as she never forgot her love and commitment to Ram, as it was very uncharacteristic of Ravan to have given her a choice. On the other hand the other POV is that Sita was the most beautiful and sought after woman and after her loud exclamations of love for Ram, Ravan's pride stopped him to carry out any violation. That is why he took her to Lanka as he wanted to make her his queen, which did require her cooperation.