Slightly different angle here (note that I'm not poly, BTW, so this is from the POV of one of the arms of a "V", not as a hinge):
The "raising children" part of marriage doesn't apply to us at this point, since we're both in our mid-40s, have all the children we're going to have (from prior marriages), and have zero interest in more (to the chagrin of my youngest daughter, who would love a brother or sister... we have cats instead

).
That said, if Chops were to marry Xena (say, for insurance reasons, or whatever), I would bow out of a relationship with him.
It is important to *me* to not put myself in a situation where I would be beholden to my metamour's good will, and legally, that is the position I would be in, were something to happen to Chops. There are cases where people have all the paperwork ahead of time to say, "Yes, this person counts too," but the hospital staff turns to the wife to say, "is this person okay to visit?" (Or, worse, they force their own morality on the situation and don't allow it period, as in some older civil union or same-sex partnership situations).
Plus, it establishes a social hierarchy that I think would be extremely difficult for many people to ignore. I'm sure Chops' mom would feel as though he picked one over the other, and I've already got enough issues with his son ignoring my existence (in favor of Xena), that I'm sure it would validate his point.
Anyway, returning to the first (and most important) point, it's not that I don't *want* Xena's good will, but I am adamant that my relationship with Chops is *our* relationship, and nobody else really gets to dictate what happens. If Xena needs something that impacts us, we talk about it. If I need something that impacts them, we talk about it. A structure that establishes a hierarchy would be extremely unwelcome to me, and I'd leave.
I know there are plenty of other people who are okay with such a situation... I just wanted to speak up as a bit of an outlier.
