Whether or not Romeo and Juliet would have regretted their marriage if they had lived would have depended partly on how it affected the relations between their respective families. If the marriage had, as Friar Laurence hoped, brought about a reconciliation between the Montagues and the Capulets, that would have influenced the young lovers to believe they had made a good choice.
An older reader who has had some experience with marriage is likely to feel that Romeo and Juliet is a beautiful love story but that the two young people are perhaps more in love with love itself than with each other and are acting overly hastily. But that is part of what is beautiful about the story. (Everything they do is reckless and overly hasty.) Only young people fall in love at first sight and give their hearts away so recklessly and totally. It has been observed that most love stories end with marriage and do not continue on into the long and often monotonous years that follow marriage. Samuel Pepys wrote the following comment in his diary:
Saw a wedding in the church; and strange to see what delight we married people have to see these poor fools decoyed into our condition. December 25, 1665
Shakespeare knew what he was doing. He was writing about young love. The deaths of Romeo and Juliet were very appropriate to this kind of story because it ends when their passion is still in what the great English poet Robert Browning called "the first fine, careless rapture!"
First love and young love are beautiful--but they don't last. Nature probably doesn't intend for them to last. They lead to having babies. That is where the relationship sort of shifts into low gear for the long climb. Romeo and Juliet had no way of knowing what the future might have had in store for them. Juliet was only thirteen years old, and Romeo wasn't much older. Some marriages are happy and others are unhappy. It is impossible to guess whether theirs would have been good, bad, or somewhere in between--but in those days they would have been bound together for life whether they liked it or not. (Nowadays, in America, marriages have about a fifty-fifty chance of lasting.)
At least Romeo and Juliet both had a lot of money. If Juliet had had a half-dozen babies, she might have been reasonably content with her life and with her marriage. Old Romeo, being the kind of man he is, would probably have been climbing over other walls and onto other balconies until he got too old and too fat to continue.