I also posted this on The Borgias spot, here is what I wrote:
There is no doubt in my mind, that he really liked Ursula a lot, but I don't think he realises that Lucrezia is his true love, albeight not the conventional love, the all powerful love that will never fate, the love of a sibling, and maybe even more.
Why else would he have ripped off his costume after the wedding?
And I am certain that he would have killed her husband, had he found out the truth. Especially seen as he killed a man "just" for insulting his mother (also so Ursula could be free of course)

@Queen_I: Beautiful answer. I do agree that he is with Ursula to fill the 'void.' What I found interesting about Cesare is he's still finding himself. There are emotions that he is aware of such as his anger, resentment, love etc and feels them acutely like his love for Lucrezia. Yet, he's still figuring out good and bad - evil and wrong etc. I think Cesare was never a naive character but he felt subconsciously bound by certain rules he hasn't broken that wall yet. A lot of his internal anger comes from this. Even though he might 'know' that loving Lucrezia more than a sister is wrong traditionally he's still ruled by his status rather than what to do for himself.

Now that he can't have her 'conveniently' he immediately looks for an outlet which was Ursula. I found it interesting that he was also attracted to a girl that seemed 'similar' to Lucrezia.

I think Cesare really loved only Lucrezia but he couldn't be with her. Ursula is fictional character, so we even don't know any woman who was loved by Cesare in real life but Lucrezia, of course DDD

99% because of this and 1% because she was good in bed.

He liked Ursula, and I think it was too early for him to understand that he really loved Lucrezia.

I think he truly loved Ursula, since at the moment he did not realize the extent of his feelings for Lucrezia, nor that they went in THAT direction. I think it is possible for someone to sincerely love two different persons, in different moments of one's life. The fact that Cesare loved Ursula doesn't diminish his love for Lucrezia imo.
But in the end, although I think his love for Ursula is sincere, I also think that his love for Lucrezia is much stronger, being born from a relationship they built over the years they spent together.