I head canon that Erik doesn't know how to play chess.
Erik has spent all of his life before he met Charles hunting Nazi's, and knowing him, he probably didn't have the resources even to play chess, only knowing the game from stories and television.
And now the very pretty man asks him if he wanted to play, and being down bad, he couldn't/doesn't wanna say no. Too embarrassed to tell Charles that he doesn't know how to play.
Charles raises his eyebrows, it's obvious that Erik doesn't know how to play, he keeps confusing the queen and king, moving the bishop in a straight line, and castle horizontal. But he doesn't say anything, letting Erik move illegally. He's very cocky and loves shoving it down people's throat but he doesn't wanna interfere, he knows Erik's past and feels out of place.
Charles lets Erik win their first games, Erik feels satisfied and relieved and you could see it in his face. He couldn't believe that all his guesses were right(That's what he thinks). But after a while, he decides to finally learn the game and opens a book.
His face turned red immediately, everything he did was wrong, he embarrassed himself in front of the very pretty man! He doesn't know if hes blushing from embarrassment or love. He was embarrassed yet the fact that Charles let him win, let him gloat. He put the rulebook away.
Their next match, this time Erik played correctly. Charles didn't wanna say anything but smile slightly.
