Has little to do with Debbie and Lisa's friendship and more to do with the MC's morals, emotions and standards. MC is still in love with Lisa, he never got over her or fell out of love with her. He can make the choice to not pursue those feelings and stay more or less loyal to his new family and his upcoming fiances, but if he's already made the conscious decision to not be with regards to dating Debbie then he's not going to go against his much stronger feelings and reject Lisa. Debbie isn't just a hook up, she's a full blown girlfriend and love interest. MC's not going to take a girlfriend on the side and then reject a different girl who he feels way more strongly about and has a history that he doesn't want to give up with.