I think it's easy to be in lust with Sancia. It looks like a lot of men were. So here's the thing I'm asking myself - perhaps being raised in these decadent times and being young, lust and play were totally enough for here. I wonder if that got different when she got older and Cesare tried to reconcile her and Joffre. After all, I can see that Joffre must have been really fond of her. The way JN showed it he is such a lost little boy and probably very impressed by her. So it stands to reason that (at least for a time) he truly loved her despite everything. A friend of mine once told me - and I'm inclined to believe it from experience - that when you sleep with someone, you get attached to them in a way and that it is a somewhat brain chemical response. Especially when your body is still growing.
And yes, I read it and.. okay, all is well and good when you write a piece of fiction. It's just that when you have some knowledge of history (or science in case of sci-fi), it is really hard to do the whole suspension of disbelief thing. And for me even more so since I'm all for CL.