Entering as a boyfriend/girlfriend on the visitor visa, or ESTA (also known as the Visa Waiver Program), is not in and of itself a visa violation. It can, however, become a problem if the girlfriend/boyfriend plans to use the non-immigrant visa to enter, and then take actions that are contrary to the intent of the visa by, for example, getting married to their US citizen love interest. This is because marriage to a US citizen implies immigrant intent, according to the US immigration laws, and traveling with what's called "dual intent" is strictly forbidden when it comes to the visitors visa or ESTA.