There is no riskiest sexual behavior, but there are a lot of risky sexual behaviors, depending on what you see as the risk. Sometimes pregnancy may be a risk couples are not willing to take. In that case, unprotected sex is risky for them. Then, at all times, there is the person in polygamous relationships with others who are in polygamous relationships too, during which he is having intercourse without protection. He is at high risk of getting a sexually transmitted disease such as HPV.