My Sim was seeing a guy and a girl and I wanted him to marry the girl so they were getting super flirty in front of the other guy that he was seeing and the other guy got really depressed and started sobbing when they were kissing lol. It was kind of hilarious actually. So like what was stated before, they can have serious jealousy issues. Moments later, my Sim and his gf got engaged right in front of the guy and he didn't seem too effected. So I think they have a good cry about it and then depending on the situation, they just get over it lol. Now all three of them are really good friends, so they put it all behind them!