Sometimes, someone else's answer is better than what I could have given, sibling marriages generation after Adam and Eve explained by Daniel Douglas

Q

Does it bother Creationists that the creation story implies Adam and Eve's children had sex with each other?
https://www.quora.com/Does-it-bother-Creationists-that-the-creation-story-implies-Adam-and-Eves-children-had-sex-with-each-other

Daniel Douglas

Catholic and former Evangelical

Answered Apr 26, 2016

I am not now a Creationist, but I once was.

There are generally four reasons for opposing incest:

• 1) unexpressed recessive genes in a bother and sister may be expressed in the child, harming the child;
• 2) incest may involve power dynamics where an older child grooms or coerces a younger child, warping the younger child's sensibilities to make sexual contact seem normal or to not object because of the lack of power;
• 3) the sexual relationship may supplant the familial relationship, making the two unable to interact as pure brother and sister because of the erotic charge to the relationship; and
• 4) even if there is no measurable mental or physical harm, it is nonetheless a violation of God's law, and thus a rebellion against God.

If we assume that Adam and Eve had many children over several centuries, rather than within a tightly packed decade or two, (2) and (3) no longer apply. Adam, Eve, and their descendants would not have been a traditional nuclear family, and it is not a given that all of their children interacted frequently with each other or would have been present before spending time together in adulthood. God commanded them to multiply and the Mosaic law did not yet exist, so (4) did not apply. And Adam and Eve were the first generation to begin the fall from perfection, so (1) would likely be negligible among their most immediate descendants.

So the idea of incest is deeply troubling now, but in the context of the Adam and Eve story and the likely structure of their family, it would be more like a morally neutral cultural difference.

Sorry for his no longer being a Creationist, but his explanation (as per empathy with his former story) is definitely a valid one./HGL