There are plenty of euphemisms for "having sexual relations" that can be used in polite or formal conversation. Some examples include "making love," "getting intimate," "engaging in physical activity," "being sexually active," or "being physically involved." More casual or vulgar synonyms include "hooking up," "getting busy," "fooling around," "getting it on," or "doing the deed." However, it's important to remember that the language we use to describe sex can have a big impact on how we perceive it and those involved. It's always best to use clear and respectful language that acknowledges the different desires, needs, and experiences of all parties involved.