For me, sexual ethics can simply be boiled down to the concept that it is ok for concenting adults to do anything as long as they harm no one. The grey area is in cases where there is hard. In those 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 homosexuals are enganing in sex. However, I find the harm to be more minimal and lacking of internal insight, that I can easily dismiss it. I find this applies to most casesof "harmed by knowledge of existance" claims.
A lot of the time, when I feel a strong emotional response towards a situation, I have to really analysize it and think on the issue. One way I do it is to imagine the extremes. If my opinion changes from one extreem to asnother, I need to understand where I draw the line and why.
As an easy example, how do I feel about underaged 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 old men coming on to old 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 harm areas 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 judgemental unless I see serious harm. For example, I know some people who have cheated on their spouse and I believe that they should not. However, I feel that is for them to decide what to do. A place where I will probably be judgemental is something like a person keeping a STD hidden from their lover.