Romantic movement (is)...a group of writers, artists etc. who followed their feelings and emotions rather than logical thought or reason...It first became popular in the late 18th century...(Longman Dictionary of English Language and Culture, 1992).
The destruction of marriage didn't start in the 1990, and neither when divorce was made easier in the middle of the 19th century. It started with Romantic movement which prioritised feelings over common sense as perfectly shown in the first novel of Jane Austen Sense And Sensibility.
As most of you probably know, Sense And Sensibility is a story of two sisters, Elinor and Marianne Dashwood. Marianne is thoroughly romantic in all her notions and has a disregard for prudence and society conventions. She is totally governed by her feelings and passions with a predictable result of falling in love with a handsome cad John Willoughby who jilts her for a wealthier woman. Elinor, on the other hand, practises quite admirable self-control and restraint.
When Dashwoods find themselves in reduced circumstances after the death of their father and have to move, one of their new acquaintances, Colonel Brandon falls in love with Marianne. He is a rich man of an admirable character but Marianne finds the whole idea ridiculous since he is 35 years old and according to her: "...must have long outlived any sensation of that kind." (p.35, Penguin Books, 1994).
When Elinor points out that Marianne who is only 17 is probably too young for him, but he could marry a woman of 27, the latter remarks that a woman so advanced in years can't hope to inspire affection in any man but could marry for the financial reason. However, in Marianne's eyes: "...it would be no marriage at all...it would seem only a commercial exchange" (p.36).
In the end of the book, Marianne marries the colonel feeling only friendship and esteem and later learns to love her husband. Their marriage, according to the author, is happy and prosperous and thoroughly refutes the romantic nonsense. This makes the book quite traditional, especially comparing to some of her other stories.
Among other opinions of Marianne was the disapproval of second affections (though her own father had been married two times) and the idea that money is wholly unimportant when one considers getting married. Elinor, on the other hand, is financially prudent. Since Edward (that's the guy she loves, for those who haven't read the book), is dependent on his mother she realises that they can't marry without her agreement and help, despite their mutual affection.
Marianne shows contempt for the rules of decorum by writing to Willoughby, even though they aren't engaged. When her sister points out the impropriety of it, she answers that she felt herself: "... to be as solemnly engaged to him as if the strictest legal covenant had bound us to each other." (p.181); to which Elinor answers: "I can believe it...but unfortunately, he did not feel the same." (idem).
As you can see, romanticism introduced the idea that legal ceremonies and formalities are of no importance, together with any financial issue which may arise and the only thing which makes a marriage valid is feelings. A couple can marry out of convenience, but it's not really marriage at all and similarly, the society conventions should go out of the window when feelings are involved. Nowadays we have couples living together who declare that they feel they are married even as they totally disregard the established way of doing it.
However, if feelings are the only thing which makes a marriage valid, then obviously when one's feelings change, it should be grounds for divorce. So it's probably not surprising that when the romantic ideas became widespread divorce became easier till finally we got a no fault divorce.
Another pernicious idea of the Romantic movement was the existence of a so-called soul mate. I have written about it before, so I won't repeat myself. I wish, however, to point out that since marriage is obviously all about your feelings and finding a perfect soul mate, then why reducing it to the union of one man and one woman? Why, anyone can be a soul mate, can't they? Why not marrying several of your best friends, your sister or your cat?
Since it's all about the soul then apparently the body doesn't matter, the marriage doesn't have to be consummated and two persons who are completely celibate should be still considered legally married to each other. Do we need any other proof that liberalism (of which the Romantic movement was the predecessor) is a modern gnostic cult?