Someone asked for a tutorial on this, so here it is. I almost never get sidetracked by sex in lucid dream, there are temptations, but usually fleeting and I forget about them as soon as I shun them.

First off: the reason that many dreams are extremely sexual is because any psychologist with an expertise in dreams will tell you that during REM sleep, males usually have an erection. An erect penis leads to sexual thoughts, there is not way to avoid this.

Sexual tension is something that builds up. The longer you go without sex or masturbation, the more sexual your dreams will become. If you are planning on trying to induce a lucid dream on a particular night, have sex or masturbate. Masturbation during your WBTB will combat the sexual temptations that your mind throws at you.

Now once you are in a dream you have to remember that this is an opportunity to do everything that you can never do in real life. Sex is real, dream sex is never as good as the real thing anyway. If you're a virgin you may be tempted to try sex in your dreams, however, since you have no experiences with sex, your mind has to try and figure things out from what it has. This requires some thinking power, if you think too hard, you will wake up. This is why often sex dreams cause you to wake up.

Even if you ignore all of the temptations that you encounter in your dream, you subconscious always wants to have sex, it's part of your ego. It will constantly bombard you with opportunities, you have to muster up the strength to ignore these. My favourite method of avoiding sexual desires in dreams is do eliminate the dream characters all together. You can either fly somewhere more private or simply kill the characters that are around you.

Think unsexy thoughts. This is the same technique you use during real sexual activities when you don't want to get too excited. It doesn't have to be sexually repulsive thoughts just non-sexual ones. Mentally do some calculus or do a mental puzzle.

The best advice is to simply avoid characters of the opposite sex though.