Pure love? Excuse me, but non-monogamous relationships can be "pure" love as well. Who is to say what is pure or not? It has only been in recent history that some places allowed people of different races and genders to marry. You're free to believe in and practice monogamy as what is right for you, but denigrating other "Non-Traditional Relationships" is patently offensive, and smacks of the religious puritanism the queer community has suffered for centuries.
The whole point of many of the routes is to explore various relationship dynamics. That's the beauty of multiple routes inv VNs - it allows for multiple explorations and creative storytelling within the same universe. Again, if something is not your cup of tea, that is perfectly fine, and there are content warnings, but as we say in furry and queer community : don't kinkshame.