Is it rude to decline a wedding invitation?

Declining is not an insult, and treating it as one has made a lot of people spend money they did not have on travel they could not afford. Couples pay per guest, often substantially, so a clear early no is genuinely useful information rather than a rejection. The etiquette that actually matters is the response itself: reply by the requested date, say no without a detailed justification, express real warmth about the marriage, and send a gift if the relationship warrants one. A short note along the lines of being unable to make it and being genuinely happy for them is complete.

What crosses into rudeness is different from the decline itself. Not responding at all is the primary offense, since it forces the couple to chase you while finalizing a headcount. Accepting and then cancelling late is worse, because the seat is paid for. Inventing a fictional conflict is unnecessary and creates the risk of being caught in it. Asking to bring an uninvited guest, or negotiating the plus-one, imposes cost on people already managing a budget. And declining while making it about yourself, at length, converts a simple logistical answer into an emotional obligation the couple now has to manage during a stressful stretch.