Yes, thinking about it is sinful
Matthew 5:28
But I tell you that anyone who looks at a woman lustfully has already committed adultry with her in his
heart
Psalms 81:12
So I gave them over to their stubborn hearts to follow their own devices
It's not everyones knowledge, but these are what the majority of the people in the world know.
3 Nephi 12
1.[28] But I say unto you, that whosoever looketh on a woman, to lust after her, hath committed
adultery already in his heart.
Mormon 9
1.[28] Be wise in the days of your probation; strip yourselves of all uncleanness; ask not, that ye may
consume it on your lusts, but ask with a firmness unshaken, that ye will yield to no temptation, but
that ye will serve the true and living God.
Esprix, in order to prove you are truly sorry for the sins you are repenting for, you have to go to the Bishop, confess your sins, but the bishop DOES NOT forgive you. Telling the Bishop is a step towards full repentance, and he can aid you and help you decide what would be the best way to repent. Also, I can’t get married in the Temple unless I’ve repented and told the Bishop. Telling the Bishop is not always expedient in order to repent, but it’s usually what needs to be done.
Yosemitebabe, thank you for doing all this extra research for me. You save me all kinds of time and headaches when you answer your own questions. You wanna become my personal assistant 
Needs2Know, I’m not here to witness. You asked for how I felt about the LDS Church. You asked that I not include “The Official stance of the Church.” That’s what I did. I can’t always clear up the surrounding alleged problems with the Church or it’s leaders. It’s hard to discern which is the truth, and which is propaganda, for both sides. I could do that kind of research for the rest of my life, and still won’t be able to answer your questions. But if you want to ask about doctrine or scriptures, or policies, I could probably help.
Esprix, I don’t have to explain myself to you or anybody, in anyway. If you are curious about the repentence process, by all means ask. But I’m not going to answer any more personal questions. I’ll provide personal examples, but at my own discretion.