Well sorry I don't know everything but I always thought adultery was any unlawful sexual act from sex before marriage to affairs. So it is just for married people then!
In reality, yes. In Christianity, no.
In Christianity, adultery equals: cheating on your partner, sex before marriage, homosexual sex (including *bleep* sex between straight couples), divorce, spilling your seed (via masturbating or using birth control), unnatural sex acts, LUST/flirting outside the relationship, etc.