In fairness to Mark Ruffalo, he may not have really known about Steve Rogers getting married in Avengers: Endgame since he admits he was given dummy scripts with fake scenes.
Mark Ruffalo spoiled Avengers: Endgame during an interview a few weeks ago.
Back at the beginning of April, Mark Ruffalo, Chris Evans, and Karen Gillan all sat down for an interview.
There are MAJOR SPOILERS for Avengers: Endgame below, so read ahead at your own risk.