IMO, if you go into this with any intention except staying together, that is visa fraud. And, frankly, as someone who has to go through all this red tape to do it right, I don't have any sympathy for anyone looking for ways to commit fraud.
Either decide to get married, or don't take advantage of the visa intended for those who are getting married.
Nobody has the right to bring a 'friend' to the US... only a spouse/fiancee.