By thought do you mean verbal thought?
If so, then I think that there are no unintentional thoughts, but they may seem so if we were not paying enough attention to their source. If we notice some feature about a person that is similar to another person we know, who we had a romantic relationship with, then we will be vaguely aware of that similarity - but if we give rise a verbal thought based on that similarity, then that thought was intended.
The time from when we perceive the similarity to the formation of the thought is very brief, and the thought overshadows the initial perception, which leaves you with the impression that the thought came out of nowhere.
The only way that an unwholesome thought is going to bring suffering now and in the future is if you form the desire for that thought not to have occurred, or to stop immediately, or to continue. If you want it to stop or not have occurred, then you will continue a line of thought about the initial thought and what it meant, and that is annoying and can give rise to self recrimination which is unnecessary. If you like the thought, or think it to be justified, then you will also continue the chain of thoughts, and might start to think that they are proper and fitting for you, which makes them more likely in the future.
The only time you are in any danger is when you feel attracted to that thought, or unhappy about it. If you feel neither and can easily turn the mind to other things, then that is best.
In terms of speech, I think you cannot speak without intention, any more than you can jump up and down. Thoughts and perceptions are also arisen by volition, and they all fall into the category of kamma.