It occurs to me that the issue with Mark, and this whole concern over it being incestuous, may simply be a smokescreen. Would Neverwhere ever be okay with you having any relationship outside of the one you share with him? He may be trying to make this seem like rational reaction when in fact it's just normal jealous BS that will probably occur with anyone with whom you would choose to date.
As for the topic of incest, it is taboo because practicing it AND having offspring decreases the gene pool, increasing the chances for genetic disease. None of these issue are at play.
I am sorry. Shoved up against the wall is something no wife should ever have to tolerate.