My friends Mandi* and Paul recently revealed that they were having a secret sexual relationship. Although I had suspected that they were attracted to one another, they always denied any involvement beyond friendship. When I asked them why they kept their relationship hidden for so long, they explained that they found their secret sex life exciting. They enjoyed the challenge of keeping their relationship hidden from others and delighted in the secret intimacy they shared.
Why was it so exciting for them to keep their sexual relationship a secret? Research supports three possible explanations:

1. New Relationship/New Partner 
When couples engage in secret sex, they are usually in new relationships with new partners. Sexual satisfaction peaks during the first year of a committed relationship and declines steadily afterward (Schmiedeberg and Schröder, 2016). The excitement of secret sex may be due in part to the excitement of a new relationship partner. Sadly, increased partner familiarity can decrease both sexual arousal and sexual desire (Morton and Gorzalka, 2015). But the allure of secret sex doesn't have to involve a new partner; keeping your relationship concealed may add to the thrill and excitement of sex.

2. Fear of Being Discovered
When we try to keep our relationships secret, the fear of being discovered may enhance our sexual experiences. In classic research, Dutton and Aron (1974) found that individuals who were more anxious or afraid (due to crossing a high, shaky bridge or anticipating painful electric shocks) found an attractive confederate more appealing. The fear or anxiety associated with their experiences actually increased their sexual attraction to the confederate. Any type of physiological arousal can heighten our feelings of attraction (see Meston and Frohlich, 2003; White et al., 1981). The heart-pounding excitement we feel over the thought of our secret relationships being discovered may actually enhance our desire for our partners. And sharing this special secret with just one other person may be important as well.

3. Sharing Secrets Increases Intimacy
We are more likely to share secrets with those to whom we are close, but sharing secrets can also make us feel closer to others. Sharing increasingly intimate secrets facilitates liking among strangers (Aron et al., 1997). Further, sharing secrets is associated with increased relationship satisfaction and relationship quality in romantic couples (Frijns et al., 2013; Sprecher and Hendrick, 2004). Sharing sexual secrets, especially about one's sexual desires, further increases couples’ sexual satisfaction (MacNeil and Byers, 2009). Sharing intimate secrets, such as a concealed sexual relationship, may increase couples’ intimate feelings towards one another.