As far as Sue's marriage with Phillotson is concerned, she becomes influenced even more, primarily because she realized that she could not bear Phillotson's sexual desire, which was, however, natural. This fact unfortunately haunts her and also partly destroys the bond with Jude. .
The theme of marriage is actually hinted before the entire plot of the novel begins. The subtitle "The letter killeth" symbolizes the existence of a marriage contract. It indirectly implies the fact that emotion and pure love should be more powerful than a written law. It is demonstrated by Phillotson's decission in the novel. When Sue leaves him for Jude, he allows her to do so, which makes a scandal. He does not act according to convention, but he realizes that this is more natural for a human being. He is consequently punished for his manners and gets fired and also Sue becomes punished later on. Illustrating this, Hardy does not want to knock the institution of marriage but he wants to point out the fact that their relationship is natural even though it is not affirmed by the particular institution (Goetz).
Arabella and Sue are both irresponsible and they scorn the act of a wedding ceremony. Arabella demonstrates this when she decides to get married to the man in Australia even though she is already married to Jude. As far as Sue is concerned, Goetz describes the situation when Jude and Sue perform the mock marriage just hours before her real wedding to Phillotson. "Placing her arm in his for the first time, 'almost if she loved him,' Sue insists on walking up the church nave to the altar railing and back down, precisely like a couple just married" (194). Among other things, this proves that Sue does not take the wedding ceremony seriously and she does not think about the act of marriage in the same way as Jude does. She "hurts the feelings of Jude, who finds her behavior here merciless" (Goetz, 194).