More importantly, why does marriage have anything to do with having children in this case? There's no reason I can see for why an incestuous marriage without children should be made illegal, as per your own reasons for why incest is unethical.
Chance of abuse or prior abuse and disorder or possible collapse of a family unit due to tension. The data is more supportive of such relationship forming as a result of abuse and power battles due to taught social points of view such as religious taboos. (western culture) If there were a set of laws allowing one sub-set of this group to marry on the grounds that it was not abuse nor a power dynamic based relationship than we would be enabling the government to review all cases of incest based marriages. After all the burden would be on the couple to challenge social and data based view points not the state. This would be a waste of taxpayers dollars and a burden to the justice system in order to provide special privilege for a minority of a minority. Also it would enable abusive based relationships to argue a case that such a relationship is not abusive. For victims such legal battles would place an unnecessary emotional trauma. In one part it would be enable people to delude themselves that their specific relationship is not abusive and have hope of acceptance via a legal battle. Due to probability based on the data of sexual abuse between family members such a marriage would be more likely be abusive there is the emotional trauma from a rejection ruling. As part of this delusion actual victims may not come forward due to coercion from the abuser based on a lack of the existing social taboo and the challenge to the legal standing. This leaves the protection basis of the ban questionable as the law is doing little to protect people. After all you are allowing people to consider such a relation as normalized rather than illegal and abnormal.
More so if one type of marriage becomes subject to a review there would be challenges to why other types of marriage are not subject to reviews as well. So for a minority of a minority gaining a special privilege could come at a cost to everyone else.
Two consenting adults is not a good argument. It was never an argument for same-sex used in as a legal standpoint but is merely pop-culture echoes. I have used it myself but typical I included legal rationale not merely a meme slogan. Borrowing from homosexual arguments for marriage does not work in the case of incest. Banning incest marriage (specific mate) does not restrict all the choices people have in marriage unlike banning homosexual marriage (category of mates).