Denobulan from Star Trek, why do they bother to get married if having sex outside of marriage is permitted?

The Denobulan species will marry three times. Each having three wives and each wife having three husbands. But, they will also have sexual relationships outside of marriage, so why bother to get married in the first place?

i + i2019-05-09T15:05:18Z

Why indeed. You DO understand
that Star Trek is all make-believe,
right? Why don't you go onto some
of the fan-fiction sites and explore
this issue then come back and let
us know how it all resolves.