... there's a surprising amount of "over 18" "incest" porn out there, and a huge amount of it is daddy-daughter - though at least officially it is stepfather-stepdaughter, with some stepbrother-stepsister thrown in.
Uncles, neighbors, and the like are much less common.
Of course, while the actors are at least 18 and there are comments in the script about the girls being 18, it's often quite obvious from the situations described that actual girls in those situations would typically be younger.
And there are certainly some reasonable concerns about incest, for biological reasons if nothing else. There are also some questions about psychological health for those involved. That said, there is no clear definition as to what relationships are or should be socially considered incest: biological parent with biological offspring? Definitely yes. Siblings? Yes.
1st cousins ... maybe? depends on the jurisdiction and the culture. (Marriage to a 1st cousin is actually preferred in many traditional cultures.) 2nd cousins? No, but deprecated some places. 3rd cousins - No problem.
In any case, it would be healthier if we could keep the two ideas separate. A preference for preteens is not a preference for family - though of course a society that strictly prohibits age disparate relationships is likely to push some towards incest as the only tenable option.