Two gay men are in one of those rare places where they can get married, and they do. They want children, so they decide to have 2 kids by (different) surrogate mothers using in-vitro fertilization: one kid using each father’s sperm. They have a boy and a girl. Twenty years later, the “brother and sister” decide that they are in love with each other and want to get married/live together, with all that that involves. Is this incest? Keep in mind that they share no genetic material whatsoever.
Does the answer depend on whether they grew up together as brother and sister, or whether they (more and more of a stretch, but it’s just a thought experiment) got split up at a young age when their parents decided they didn’t want to be together after all, got divorced, and never saw each other again?
ADDED: What about for adopted kids, who don’t have any link whatsoever, even to the respective parents?