Individuals can draw the line wherever they please in their own behavior. The question is where government is allowed to draw lines for everybody, and what reasoning those legal restrictions are based on.
Personally, I think incest is ewwww but I know of no valid reason for legally prohibiting it between consenting adults, any more than any other (nonviolent) sexual acts should be legally prohibited between consenting adults. (However, it might be appropriate to scrutinize such relationships more closely for confirmation of valid consent. For example, a parent who’s kept a dependent child emotionally stunted and helpless until the age of 18 and then begins a superficially “consensual” sexual relationship with the adult child should not get away with that kind of exploitation.)
Couples who form incestuous relationships without knowing they’re incestuous, on the other hand, will probably be perceived as more sympathetic. If, e.g., two siblings separated in early childhood meet as adults and fall in love without realizing their biological relationship, then it doesn’t seem appropriate to stigmatize them as criminals when they do find out about the relationship, in addition to all the other traumas that the realization will doubtless create for them.
So, in principle I’m against the criminalization of incest between consenting adults, although in practice I don’t think it’s a significant social issue, because very few people actually want an incestuous relationship.
In principle I’m also against the legal prohibition of incestuous marriage between consenting adults, because any unmarried consenting adult should be able to marry any other unmarried consenting adult, at least as far as governmentally recognized civil marriage is concerned. (Let’s leave the whole question of polygamy aside for now and just consider what’s a fair and consistent policy for monogamous two-person marriage.) There should be scrutiny of genetic relationship for understanding possible reproductive consequences, as there is scrutiny of hereditary diseases for that purpose, but I don’t think it’s the government’s business in this modern contraception-enhanced world to permit or forbid civil marriages on the basis of possible reproductive consequences.
But in practice, I think incestuous marriage is even less of a significant social issue than incestuous sex. One of the functions of civil marriage, as noted above, is to provide two unrelated adults with a recognized familial status, so they can be each other’s next of kin, etc., for legal purposes. Incestuous partners already have a recognized familial status, so that aspect of marriage isn’t as important in such cases. And again, there will probably always be only a vanishingly small subset of people who actually want an incestuous marriage.
So my positions on incest-related legislation stack up more or less as follows:
Decriminalizing incest between consenting adults, with appropriate provisions, where it’s not already legal: Would vote for, would not actively promote or advocate unless I had reason to think that some people were being genuinely adversely affected by the current status of the law.
Criminalizing incest between consenting adults where it’s not currently illegal: Would vote against, would actively protest if I knew of people who would be adversely affected by it.
Removing prohibitions on incestuous marriage between consenting adults, with appropriate provisions: Would probably vote for, would not spontaneously advocate.