This is very bad form. The only thing worse is turning up after not RSVPing.
Firstly, weddings are expensive undertakings. Every guest comes with a cost. Secondly, if a space opens up, you'd best believe the families will want to fill it with someone else.
As soon as you know that either you or your plus one can't make it, you have to tell the person who invited you so they can take care of logistics and make any relevant alterations to seating charts.

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