How do you politely decline a plus-one you were offered?

Short answer

Reply promptly and thank them, then say you cannot accept the plus-one because the couple has kept the day intimate. Offer a warm alternative so they still feel welcome.

Reply as soon as you know, so the couple can adjust their seating and catering. Start with thanks for the offer, then be clear and kind about your decision. You do not owe a long explanation, but a short reason helps.

A good message might say that the couple has chosen to keep the wedding small and that you are sorry to miss out. Then offer something to soften the news. You could suggest meeting for a drink after the wedding, or invite them to the evening reception if that is open to all guests.

Here are a few ways to phrase it:

Thank you so much for thinking of me. The couple has kept the ceremony to family and closest friends, so I will not be able to bring a guest.
I am so sorry to miss the big moment. Let us grab a coffee soon so I can hear all about it.
If the evening party is open to everyone, I would love to join for the dancing.

If you are worried about a partner feeling excluded, check whether the invitation explicitly names them. If it does not, the couple has made a deliberate choice. Be gentle but firm, and do not leave it until the last minute.