First, I came in to polyamory after being married; poly is the (relatively) new development in our relationship. (We were married this last summer, poly stuff came up this October or so.)
However, I think I'd go for marriage even with poly. To me, marriage is a public declaration that we are committed to doing our very best to make this relationship work for life. It's not a decision that I made quickly, or one that I take lightly. If I were to have a long-term poly relationship with second person that I felt willing to make that commitment for, I would want to have some kind of marriage ceremony or decision (although it obviously wouldn't be legal and, because of job / family issues, may not be quite as public).
Of course, the social bonuses of marriage are nice - I was getting sick and tired of being treated badly when I tried to help Mal with doctor's appointments / school business. Receptionists etc seem to be much nicer when I can tell them I'm his wife (as vs his fiance / girlfriend). Might get interesting if I find a female partner and tell them I'm her wife lol.
Keira Raven, married to husband Mal, interested in a woman for a triad or vee
(Previously known as Dakota Raven, husband previously known as Adam)