Are thoughts ever sins?

According to the Bible, absolutely (e.g. even looking at a woman with lust in your heart is considered adultery). 

Which I find pretty messed up; it’s basically making thoughtcrime a thing.

That sounds like an excuse to sexualize and objectify another person.

@progressofthepilgrim So long as it's all in your head and it doesn't affect someone by being turned into an action what goes on in your head is no one's business but your own.

To say that you've cheated on your spouse if you've ever thought someone other than them is hot is ridiculous.