No, an adulterous thought is not a sin. It's just a thought.
Frankly I find the whole notion of sin tiresome, pointless, and meaningless. I don't really think anything's a sin, the word has no referents for me. It's usually defined as a transgression of some divine or spiritual law, and since I don't believe there's any such things, there are only human ideas that people try to elevate to that status, the concept is incoherent to me. People do make mistakes of course, of greater and lesser degree, and I've certainly made my share over the years, but sin? No such thing, in my view. The word has no real meaning outside its religious context, and I rejected all religious belief years ago as superstitious nonsense.
Besides, when you stop looking at attractive people and having certain thoughts about them, it can only mean you're dead, which in turn means, granting that there is such a thing as a sin, that to be human and alive is to sin. Many religious traditions teach precisely that, and I have no patience with that idea. It's purely a manipulative guilt trip, it teaches people that they're intrinsically worthless, or even less than worthless, they're actively evil, by themselves, which I think is dangerous, stupid, and very damaging.
And double besides, no judicial system in the world that I've ever read about, through all of human history, has ever maintained that you can condemn people for what they think, you can condemn them only for what they do, which might include expressing their thoughts. Only religion condemns people for what they think, on the comprehensive grounds that the deity knows what you think regardless of what you do or don't do about it, and I have nothing but contempt for that idea.