While it may not technically be illegal is some places, it still is and always should be immoral.
I don't tend to look at people's AVN preference through a moral lens, this shit is all a fictional fantasy. I assume that most can separate fantasy from reality. I just find it funny how far things have to go for it to be not seen as vanilla. Or just normal non-related romance isn't enough for some.
If your position is based on genetics, I'd like to state that when people argue that first-cousin relationships are immoral primarily because of the risk of genetic defects, that risk is often overstated. For a single generation, the risk of congenital conditions is estimated at around 5-6%, compared to roughly 2-3% for unrelated partners - similar to the baseline risk associated with a woman having children at age 40. This also assumes reproduction is even intended, rather than prevented by a couple through measures like sterilization or contraception.
To me, arguments against incest that rely heavily on genetics risk smuggling in eugenic assumptions - especially when comparable or greater risks are widely tolerated in other contexts. We generally do not treat elevated genetic risk, by itself, as a moral failing of a relationship. That doesn’t mean all incest is morally permissible, but it does suggest that genetics alone is an ethically unstable foundation for condemnation, particularly in the case of cousins.
When we’re talking about consenting adults, I’m generally wary of categorical moral prohibitions without a very strong, non-selective account of harm. In the case of first cousins, most of the usual moral concerns associated with incest such as coercion, power asymmetry, and role violation don’t reliably apply, leaving genetic risk to do most of the work.
Combined with the fact that cousin relationships are far less likely than parent-child or sibling relationships to involve coercion or abuse, this helps explain why cousin marriage has historically been treated differently - both morally and legally.
In any case, you're right in that in AVNs it's all fictional fantasy anyways. But it's also why someone like Sadie doesn't bother me morally.