The phrase "have relations" can be considered quite formal and is often used in medical or legal contexts to describe sexual activity. However, there are many more colloquial or slang synonyms that can be used in everyday conversation. Some common examples include "hook up," "get it on," "make love," "have sex," "sleep together," "fool around," and "bump uglies." These phrases can vary in their level of vulgarity or intensity, so it is important to consider the context and audience before using them. Ultimately, there are many ways to describe intimacy between consenting adults, but it's important to always use language that is consensual and respectful.