I know a lot of writers who struggle with writing about uncomfortable topics, be it sex, other bodily functions or what have you. I don't think any of these things should be left off the page all together, but as a writer you need to be conscious of why you're including it and when. Is it adding to the plot of the story or getting in the way?
If you go into too much detail about the wrong topic, it will take your reader out of the story and make them less interested in reading the rest. Recently I read a novel where a character was eaves dropping on someone from inside of a tour bus bathroom. While some of the things in the scene were funny and plot-driven (the information she gained was important to what happened later) other things brought me out of the story. I had to skip over the parts where the narrator described the sound of her urine hitting the water in the toilet bowl, because as a human being, I've heard the sound before and don't need it described to me.
Don't think I'm being prudish -- I'm all for descriptions of any kind that are in some way relevant to the plot of the story. In a tense moment like the one described above, the sound of the narrator's urine is not directly related to the plot, it just gets in the way of an otherwise suspenseful scene.
Most of the time things like this can be left off the page. You can trust that the reader will know what's going on without having it spelled out. There is a time and a place for these sorts of details, but they have to be carefully chosen and should always add something to the plot rather than simply filling space.
What sorts of things do you leave off the page? How do you decide what stays?