If you were to marry Nico Robin, you would have to accept the reality that she's basically never truly grown up despite being in her 30s and that her maturity is just a front to get by and cope with her traumatic past and lack of a real childhood or things like friends growing up. If you married her, she would want to do all kinds of childish things like ride roller coasters and play dress up and eat candy that she never got to do and you would have to deal with it.