I'd have to say none of the above.
Reasons being?
Love
Love doesent work because love is inherently a fluxuating kind of thing. They way you love someone in romantic love will go away, though in any kind of relationship it re-sparks over time. I've noticed in the best marriages, the partners acquiesce (conciously or not) that they are good friends, or people who work well together with reccurring bouts of romantic love. Love is fickle, and taken into account can mean a better marriage (or any kind of social dynamic really).
Religion
Being atheist, though raised catholic and studied in religous matters, I don't really see the point here. Sociologically speaking, its a matter of "we share the same beliefs, therefore we gain comfort from those communities of beliefs" and nothing more than that. If you within that belief, youll attribute divine sanction, or what have you, then of course it would be more important to you. But it cannot stand alone.
Tradition
Pretty much the same as Religion, sans the spiritual conotations. Cannot stand on its own, and on its own might do more harm than good.
Money
Pragmatism embodied, as a qualifer with the other 1(or 2 or 3 depending), it can make a difference dependent on the views of money each partner shares. Money does lead to comfort which leads to more time to get to know one another (which may be good or bad). This one is kind of a wild card, because you never know which way it will go.
All this being said, I believe binding monogamous relationships to be constucts of fear, fear of being alone, fear of losing someone.. and on that note, i find them to not be an appealing choice in general for the level headed pragmatist.