Polygamy/multi-partner marriages of all varieties are perfectly legal in UGC space. Laws may vary by planet.
Reaha's mom is. See below:
I don't think Reaha even knows who her dad is, much less was he living with them. All of her siblings have different fathers.
My personal head-canon is...
Marriage is probably vanishingly rare (and almost always open to some extent), but it's not outlawed or anything. Besides, married couples could always move to NT after the fact.