"Marry" here means to have sexual intercourse; as is indicated elsewhere, the black population's concept of marriage is rather different to the white interpretation. For a white girl to sleep with a mixed-race man was totally taboo, although many white men slept with black women. As Rhys herself said, it was:
'in those days [the 1900s], a terrible thing for a white girl to do. Not to be forgiven. The men did as they liked. The women – never.'