You want an explanation why's incest bad? Ok. I am going to explain it to you the best way I can, as an incest survivor.
1) Abuse. Most, if not all, of the time, incest is abuse. I do not think I need to explain why abuse is bad.
https://pubmed.ncbi.nlm.nih.gov/3766794/
https://pubmed.ncbi.nlm.nih.gov/1592946/
https://en.wikipedia.org/wiki/Incest
https://en.wikipedia.org/wiki/Child_sexual_abuse
2) Power imbalance/dynamic: In incest, there's always a power imbalance/dynamic. And don't say there's none between siblings, everyone who thinks just a little bit knows it's just not true.
https://www.psychiatrictimes.com/view/ramifications-incest
https://journals.sagepub.com/doi/10.1177/1079063214558941
3) What if they are twins? Well, most of the time, incestphilia is disordered or traumagenic. Like almost all paraphilias. And there is little to no chance that these two siblings actually love each other. They could have been conditioned or just misguided by their paraphilia.
https://en.wikipedia.org/wiki/Traumatic_bonding
https://www.researchgate.net/publication/257553129_Sibling_Incest_Dynamics_Therapeutic_Themes_and_Clinical_Challenges
4) What if it's between two adults and it's consensual. We can never know if something is actually consensual. Also, don't forget the medical, genetic risks.
https://en.wikipedia.org/wiki/Incest
https://www.bbcearth.com/news/what-are-the-effects-of-inbreeding
5) "I am an adult && with someone in my family and it's not abusive or whatsoever!" I don't believe you, at least on the not abusive or whatsoever part. But I know that I can't change your mind like that. If you don't like me, block me.
https://en.wikipedia.org/wiki/Traumatic_bonding
https://www.psychiatrictimes.com/view/ramifications-incest
6) What would you do if you see two family members actually love each other without abuse or anything? I still would find that incorrect or wrong, but I do not harass people.