Looking back, I don't think Hallownest or Pharloom has a concept of marriage.
Now, the idea of a (mostly) monogamous long-term romantic pair is not unusual. The Pale King and White Lady, the Green Prince and his partner, the Grey Mourner and Traitor's Child, Plinney and his partner, plus various animals that mate for life. These sorts of unions are familiar and respected by the wider populace. At the same time, most characters don't have a romantic partner, but its unclear if that's the standard for these societies or a symptom of their respective declines. After all, every pairing above has at least one member already dead.
But its telling that the Green Prince says "partner" and "lover", but never "husband". Characters refer to a "mate" or "partner", but never "spouse". In fact, the only time "husband" or "wife" is ever said in-game is in reference to Cornifer and Iselda, who came to Hallownest from an unknown location. And they use it a lot! It's quite striking how casually they use these words that are never seen elsewhere.
And more importantly, the Pale King and White Lady are never referred to as married. They are king and queen, have a "union", and are obviously in love, but unlike Cornifer and Iselda they never use the words "husband" or "wife". If the king and queen are not married, then however Hallownest signifies these relationships is very different to the way most humans understand it.
Overall, it's clear that while certain bug cultures do engage in marriage, it's not universal. And the fact that two outsiders are the only ones to mention it indicates that Hallownest and Pharloom are not among them. However these cultures understand and value romantic partnerships, it's markedly different to human ideas of legal and societal marriage.
(Side note: the only times "mate" are used is in reference to animals like Obblobbles and the Great Conchflies, or Hornet and Hunter in the journal entries, talking about how they don't have a mate. Also Crull talking about Benjin. So mate in the romantic sense isn't common either.)