Marriage is many things in Tartuffe. It's political; that is, it's just as much about making alliances as it is about love. It is about love, of course, but the workings of love are hampered throughout the play. We're told that marriage is ultimately decided by the father of the bride. We're also told, however, that a father's mistake will cost him dearly, that the bonds of marriage are only as good as the match that's been made. All this is to say that marriage is important. Without the drama between Orgon, Mariane, Valère, and Tartuffe, there would basically be no play at all.
In Tartuffe, marriage functions as both a political tool and a manifestation of true love.
In Tartuffe, Molière introduces a marital conflict into the play in order that he might explore issues related to it, namely adultery and the role of women in society.