Very often a couple will start off with the idea that they don’t want children and then have that belief challenged down the line when faced with things like a surprise pregnancy, a failed vasectomy, a realization of ones own desires etc.
Governments make laws based around scale. So needs of the many vs needs of the few, and the risk of people subverting the process and causing unimaginable pain by giving birth to children with serious defects IMO outweighs the benefit of those who would honor the process.
Besides the taboo nature of an incestuous relationship, it can have real impact on a governments role in social programs and acting on the best interests of its citizens.