Charles Manson can "marry" someone. The lady in question this time around seems assured but vacant to me, and her father suggests she is creative with history. Another article says prisoners have the constitutional right to marry (and tries to show what some of the motivations are(*)), but Manson is in for life and nobody is going to parole him--there is no question of making a life together. Even Manson admits this is a fake, and it is not the first prison pseudo marriage for him either. Why cooperate? A minister who thinks a couple isn't serious or isn't ready can refuse to conduct their ceremony. Make them rely on Flying Spaghetti Monster or Universal Light mail-order ministers.
(*) Apparently different criminals attract different sorts of fans. And having a "wife"/"girlfriend" is a status thing inside.