There are many different synonyms for the term "sex activity" which can be useful when trying to communicate in a more nuanced or coded way. Some common options include sexual behavior, intimate relations, lovemaking, carnal relations, physical intimacy, erotic actions, sexual contact, and sexual encounter. Each of these phrases carries its own connotations and implications, and can be useful for talking about different aspects of sexual interaction (such as the emotional or physical components). Other possible synonyms might include euphemisms like "hooking up" or "getting together," or more explicit terms like "fornication," "intercourse," or "coitus." Ultimately, the choice of synonym will depend on the context and the audience.