It is okay to get aroused by your own sex scenes.
In fact, it's pretty much required. Remember, part of the intent of a good sex scene is to arouse the reader. And you're not likely to do that unless you, yourself, are feeling the same delicious tremors. You should be envisioning what you're writing and - whether with one hand or two - transcribing these visions in detail.
Contrary to popular belief, people think during sex.
I know this is going to be hard for some of the men in the crowd to believe, but it's true. The body may race when it comes to sex, but the mind is also working overtime. And just what do people think about? Laundry. Bioterrorism. Old lovers. That new car ad. Sex isn't just the physical process. The thoughts that accompany the act are just as significant (more so, actually) as the gymnatics.
If you ain't prepared to rock, don't roll.
If you don't feel comfortable writing about sex, then don't. By this, I mean writing about sex as it actually exists, in the real world, as an ecstatic, terrifying, and, above all, deeply emotional process. Real sex is compelling to read about because the participants are so utterly vulnerable. We are all, when the time comes to get naked, terribly excited and frightened and hopeful and doubtful, usually at the same time. You mustn't abandon your lovers in their time of need. You mustn't make of them naked playthings with rubbery parts. you must love them, wholly and without shame, as they go about their human business. Because we've already got a name for sex without the emotional content: It's called pornography.
BONUS STEP!! STEP 13!!!
Read the Song of Songs.
The Song of Songs, for those of your who haven't read the Bible in a while, is a long erotic poem that somehow got smuggled into the Old Testament. It is the single most instructive document you can read if you want to learn ow to write effectively about the nature of physical love.
I am not making this up.
So, there ya go. 13 steps on how to write a sex scene.