Maybe he just felt that you would be disappointed in him since he’s with someone else’s girlfriend. Even though you may not judge his actions, his conscience may already be judging them. Whether you know the girlfriend and her boyfriend aside, maybe he just didn’t want to look like less in your eyes. Sometimes we do things that disappoint ourselves and maybe he feels like a piece of crap already and just couldn’t bear to have you know his shameful (I assume shameful anyway) secret.