You appear to be very cynical about marriage and relationships in general. Visas are granted to the spouses of US citizens because the US recognizes the right of these citizens to have a family life. It is not only the spouse who is harmed if the visa is denied but also the US citizen in question, whose rights the US government is obliged to recognize. Utilitarian considerations are largely irrelevant. As far as justice is concerned, I don't see why you think it would be unjust to grant visas to spouses. Of course spouses of US citizens are not the only people who should be granted visas, but no-one suggested that was the case.