If you do not know you were cheating, is it cheating?

Lets say a guy meets up with identical twins, those that dress alike, style their hair alike, etc. The guy gravitates to one twin over the other and starts dating her, but her sister wanted him too. If the dude goes over to the twins home, slightly drunk already expecting to meet the twin he chose, but runs into the other twin; but don’t realize it. The twin he wanted to see got called away because of some urgent matter and told her sister to tell him that. If she doesn’t, but invites him in, get him more drunk and they end up boinking. If he thought the twin he had sex with was the one he was dating, did he cheat? Even if he didn’t know, or learned after the fact is he still a cheater by proxy, you know, ignorance is no defense?