Why is homosexuality a sin?

Other sins are easy to see why they‘re sins- stealing harms the owner of that item, murder hurts the person murdered and possibly others, but why homosexuality? If 2 men are happily in a relationship, who is it harming? If 2 women kiss alone, who is negatively impacting? Was it mistranslated?

(I am fine with being a sinner, btw. I’m against Christianity and also practice witchcraft. So dont try to “save me” in the comments.)

Edit: I’m asking why homosexuality is harmful