“与某人发生性关系”常见的英文表达有 “have sexual relations with someone”“have sex with someone” 或者更正式、委婉一些的 “engage in sexual activity with someone” 。
例如：
He admitted to having sexual relations with her.（他承认与她发生了性关系。）
It's illegal to have sex with a minor.（与未成年人发生性关系是违法的。）
They engaged in sexual activity with each other last night.（他们昨晚发生了性关系 。）