So, you assumed that showing a romantic interest in your game doing sex acts with another male would be a good idea? Netorare doesn't have anything to do with being in love. It is being romantically interested in. If the MC in the game saw Laura having sex, would he be upset? If you were interested in a girl and you had sex with her and then you saw her having sex with another guy, would you accept her excuse of, "I never really loved you, I was just using you. You're dumb for assuming otherwise." Because that what you have done. Player assumed *no* girl would have sex with others because the game said it was a lighthearted harem.
*Any* cheating on the main character is too much cheating and is Netorare and it will forever brand you and this game as that. Harem fans will not trust you to not do this to other girls now. You said that soon the MC will steal a married girl, now players will fear that before the MC steals her, he might be forced to watch her have sex with her husband. Or she may go back to him after the MC has her. You will lose player trust and fans of Netorare will not support your game because it only has the one scene. You added this in for nothing and it might cost you everything.
And again, you get to decide how succubus in your game act. She could have been loyal to him if you wanted her too. You could have made her loyal, you made her betray the MC.