Ok, serious answer. Reproduction taken out of the picture, the main issue with incest is relationship dynamics, and the idea that your blood family is a group of people who were fundamental pillars of the development of your personality and identity as you grew up (or you are to them), which means that a sexual or romantic development falling out could shake those pillars at best, or there could be an almost inherently abusive power imbalance at worst. You could find situations where a technically incestuous relationship wouldn’t be a terrible idea (for instance, step siblings related due to half of their parents getting divorced and marrying someone else, who have only met each other after their adult persona already has solid foundations), but it generally almost always is terrible.