For me, sexual ethics can simply be boiled down to the concept that it is ok for consenting adults to do anything as long as they harm no one. There are grey areas. In the grey areas, I apply the golden rule (rule of reciprocity) to all involved to decide if it is ethical.
For example, conservatives complain that they are harmed by the knowledge that gay people are engaging in sex. However, I find the harm to be more minimal and lacking in internal insight, that I can easily dismiss it. I find this applies to most cases of "harmed by knowledge of existence" claims.
A lot of the time, when I feel a strong emotional response towards a situation, I have to really analyse it and think on the issue. One way I do it is to imagine the extremes. If my opinion changes from one extreme to another, I need to understand where I draw the line and why.
As an easy example is, how do I feel about underage sex? My first reaction is say it is wrong because they need an adult mentality to deal with it well. But I remember being young and wanting sex... Am I being unfair because I know it doesn't relate to me anymore? So one thing I do is see where the taboo lines are and why.
One major area is age difference. Society is more ok with a 17 and a 16-year old having sex than a 35 and a 16-year old. Why? We worry about the younger one being manipulated easily, since they do not have a well-established sexual identity. One example of this is from the Twilight series. My wife hates older men coming on to younger women, but in that book a 90-year old guy wants to have a relationship with a young girl in high school. That did not bother my wife, so I asked her why. She said that the book did a good job of portraying the man as a 17-year old boy. In other words, he did not manipulate her with his 90 years of knowledge.
Incest is a tough one, as well. The potential for harm is that it could be a continuation of a childhood sexual identity issue, and genetic problems with children. As an example where I was ok with incest, I heard about a man and woman getting married and later finding out that they were really siblings. However, they grew up as strangers. So to me, as long as they did not have biological children, it would be ok. In reality, I believe they annulled their marriage.
But that being said, I am not overly judgmental unless I see serious harm. For example, I know some people who have cheated on their spouses, and I believe they should not. However, I believe that is for them to decide what to do.
A place where I will probably be judgmental is when a person is keeping an STD hidden from their lover.