The Sridei Eish Vol 2, 8 rules that by gatherings, even which are not for matters of sanctity, men and women should sit separately so as not to mix.
The Kitzur Shulchan Aruch 149:1 (based on the Bach and Beis Shmuel) rules that “shehasimcho bimoinoi” is not said during the benching of a sheva brochos if men and women are seated in the same room because there is no simcha when the evil inclination is active. This is one of the basis that many contemporary poskim use to rule that there must be a mechitza at weddings and sheva brochos.