Let's be honest. I write epic fantasy, urban fantasy, and sci fi. Inevitably I'm going to have to write fight scenes and possibly, if I feel like it, the occasional sex scene. I went looking for some helpful advice on how to do just that. And I found it.
The first helpful article I found was this one. The writer of the post interviews two authors who have real world fighting experience about how to write a significant fight scene. One of the pieces of advice in this article is to go take a martial arts class. Or if you can't, spend a great deal of time researching different fighting styles. They also bring up the point that you don't have to mention every move by name. Being vague isn't always a bad thing in fight scenes.
Then there's this one. This is the first article I ever read on writing fight scenes. Some of the advice it gives is setting the scene before the fight begins. Have the characters visit the spot in an earlier chapter. They might not remember all the details but they're going to remember something. Another bit of good advice is about sentence structure. You don't want all of your sentences to be short. A few longer sentences with good, strong language works as well as the short sentences.
Now in regards to sex scenes, I found this article by Diana Gabaldon on writing sex scenes. In it she discusses the Rule of Three - if you use three of the five senses, it makes the scene three dimensional. She also brings up the point that sex scenes are as much about the emotions as about the actual physical activity of sex.
In this article from the Huffington Post, the author describes the conundrum of figuring out how much is necessary. He also talks about using appropriate language. He talks about how sex can be a physical failure and an emotional success.
This and this offer even more links to valuable resources. Take some time to research what these people have to say. You might be surprised at how much they help your writing.