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Don't want to give advice that later might come back to burn you, but I know several people who have married for citizenship and it went without a hitch. (Unfortunately gays and lesbians still have to do this if they want to stay with their partner). They were all friends, so knew a lot about each other, and weren't nervous or uncomfortable during the interview. They were all South American, by the way. I think how easy or difficult it goes will depend on what country the guy is from and how well you know him and how comfortable you can be during the interview.
My friends NEVER had to have sex with their purported mates. You need to remind yourself that lots of people get married who don't belong together - people who fight incessantly, never have sex, etc., and they all can get citizenship - it's not for immigration to decide how in love you are, just that the marriage is what both people want.
Sorry, but the advice for the LW to consider prostitution instead is a little silly.