Intentional or unintentional sexual thoughts I've been restless for these past few days. Once I have an unappropiate thought about a holy figure, and I think it was unintentional (not sure though). But then I kept thinking about it over and over again (I tried to analyse whether it is intentional or not). My questions are:

Is sexual thought about holy figures unforgivable sin?
Am I creating bad karma by thinking about it over and over again?
What advice do you have for me? I keep on feeling really guilty and keep on thinking about it.
Random thoughts (like random sexual thoughts) don't collect much bad karma. Intentional and deliberate thoughts generate karma. Habitual intentional thoughts accumulate karma.
Now, you are feeling remorse because you had those thoughts.
What you should do is acknowledge that those are random thoughts. Acknowledge that you don't have evil intentions.
And now you should disassociate from those thoughts using mindfulness. Just watch those thoughts as they appear but understand that they are not related to you or anything. Just watch and acknowledge it, then let it go.
Watch this video talk on YouTube on the scientific research done in this area.
From a canonical perspective, the Lonaphala Sutta explains that a trifling evil act has significant consequences for an undeveloped person but hardly any effect for a developed person.
Your remorse is part of that unintended significant consequence. But if you learn to let go, then you would have developed your mind to detach from such trifling negative thoughts.