looking at other guys can lead to a threesome too. Like happened in this update.
Just MFF is "nice" - and MMF is a tabu
Some males are just so arrogant and think sex is only about what THEY want -> SHE has to agree
Man: i want you to have sex with me involving another woman. She: "But i am not gay" - he: "Bi is not gay - you should be ok with it".
While on the other side:
She:" i want you to have sex with me involving another man" - He: "NEVER - end of discussion!"
To be fair, its more a biological drive, harems involving one man and multiple women have been culturally, and biologically relevant for the entirety of not only humankind but also most of animal kind (more specifically mammals). Men that are alive today are the descendants of animals that won the genetic game, while most cucks ended up dying out in the gene pool, its genetically driven, the jealous men tended to mate guard and have offspring while the men that were into sharing did not get to pass on those genes, because that genetic drive ends up deleting itself in the population long term over millennia. Its interesting to think about it anthropologically when i see these discussions, and it explains why on average more women are ok with other women, and most men are not ok with multiple men, the genes for this end up being selected over time due to sexual practices of our ancestors.