Sexual misconduct may be a simple definition, it is having sexual relationships with another person when it is not proper. I think the Buddha listed things like having sex with someone who is underage, having sex with someone who is not free from obligations to somebody else, which could include a daughter of a parent or a wife of somebody else, etc. Having any kind of sexual relationship that would harm somebody would be misconduct. What is important is actual physical and verbal expression, not if it's just thoughts. When we think of morality, it is really the physical and verbal expressions that we are going to judge. The thoughts of greed and whatever, whether it is regarding sex, whether it is regarding food - they come under the term greed, but it may not be sexual misconduct. The minute it comes out in your speech and in your actions, then that is where sexual misconduct starts.
But more important is to actually know what types of sexual misconduct there are. Most important is are you harming anyone? That is very important, and it is not just in this moment, you might think it is so much fun in the moment - like every affair that someone has, a guy with another person's wife, or a woman with another person's husband, in every affair, they think it is great while they are having it, but later when things go wrong, it's not that great anymore. Recently in America a presidential candidate was having an affair during the election campaign. Everyone thought he was a great guy, he really had this image of being ok, proper, the good guy, and later, wow, it all just came out and it is so much Dukkha, it is incredible. People don't often think of this when they are having the affair - it is great, it is fun, they don't think of it as Dukkha, they don't think they are harming anyone.
This is part of understanding the Law of Cause and Effect, when you are thinking what is sexual conduct, you have to look at what is going to be the result of the conduct.