I strongly suspect that if incestuous marriage had been legal and socially accepted, Ivanka would be the de jure, rather than merely de facto, First Lady.

Legalization of incestuous marriage (by completely repealing Code of Virginia § 20-38.1) will benefit families by expanding the pool of available mates to include those within the family itself. One difficulty that sometimes arises with finding a suitable mate is information asymmetry, in which one does not know enough about potential mates to make a good decision. In familial relationships, they mate tends to be more of a known quantity.

A commonly-cited disadvantage of incest is the potential for birth defects in their offspring, but this is usually not a major problem if the two are genetically distant enough (e.g. father and daughter as opposed to brother and sister). The major problem arises if inbreeding is carried on for too many generations.

In the manosphere, it is sometimes said that young women desire a father figure (even to the point of looking for a man who will spank them and/or have them call him "daddy"), and that because of sexual differences in emotional attachment, the only truly sacrificial love that a man can expect from a woman is from his mother. It also seems logical that a parent might feel some attraction for a child who resembles the person they fell in love with and had children with. The potential to have incestuous relationships down the road could provide an incentive to produce offspring and to care for them as an investment in one's own sexual future.

A father will tend to be significantly older than his daughter, and therefore will be more likely to be a better provider, and less likely to beat her, than a man her own age.

As always, we can rely on natural selection to ensure that people have no greater tendency toward incest (or any other activity) than would be optimal.