you're playing a harem focused game, and one of the most common fantasies for guys is having a threesome.
the dev is going that route thinking that if most like it, it shouldn't really matter.
you can just skip the scenes that you don't like too.
in universe, it can make sense. the MC is a womanizer. that scene is for Aelinia to push her political agenda, and for Lydia to put in your head the idea that you can be king. and a good way to convince a womanizer is having a threesome. if you like one of the girls, then she "gifts" you the experience. if you like both, then they share you willingly. they are mostly focused on the MC though. if that helps
![Big grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
what doesn't make sense in that scene, though, is that there can be cases where one or both can still be virgins when offering you that threesome. and knowing both characters, they would never have their first time like that. so it should really only happen if you took both their virginity, so if you're on both paths at the same time.