Flowering is a euphemism for a girl’s first menstrual period. The blood is the seal of womanhood. It means that she is now fit to be wedded and bedded.
In the general Tamrielic view, girls may well be wed before their first flowerings, for political reasons, but it would be considered perverse to bed them. And such early weddings, even without sexual intercourse, remain rare. Generally weddings are postponed until both the bride and the groom have passed from girlhood or boyhood to maidenhood.