What did Swami Vivekananda say about marriage?
Swamiji was not at all against marriage. But He disliked marriage to take place just from mutual attraction and love, instead considered it as something sacred and inviolate as is evident here (search page 180) from a letter to a Western lady:
In my opinion, a race must first cultivate a great respect for motherhood, through the sanctification and inviolability of marriage, before it can attain to the ideal of perfect chastity. The Roman Catholics and the Hindus, holding marriage sacred and inviolate, have produced great chaste men and women of immense power.
On another occasion, it was found that to promote unity among the Hindus, he encouraged intermarriage between castes and sub-castes (page 140). But he also held that ideals and rules of a monk's life cannot be applied to a householder's life as he writes in a letter to one of his American sisters Mary Hale who was full of idealism (page 111):
Therefore my advice to you is that until you bring down your ideals to a more practical level, you ought not to marry. If you do, the result will be misery for both of you.
Therefore, though himself a monk, Swamiji encouraged marriages but with absolute loyalty and devotion between husbands and wives.