Sex dreams are pretty common.
About 70% of the women and almost all guys have sex in their dreams (once in a while).
It can be a very realistic dream, having sex with a guy or girl you are in love with. But dreams can also be outlandish, doing things you would never engage in in real life.
Dreaming about sex can lead to orgasm, but for most people this is a rare occurrence. Only 5% of the guys and 1% of the women climax in their sleep more than a few times a month.
Sometimes people feel guilty about their dreams, especially when they are in a steady relationship but during the night have a hot encounter with someone else. But one should realise that you don’t invent your dreams: they happen to you. So there is no need to feel guilty. Moreover, these dreams don’t harm anybody.