In Shakespeare's time, "romance" had a different meaning. Romance did not mean a love story, romance was a drama that had a happy ending. Romeo and Juliet was a tragedy. Had Shakespeare intended this to be a romance, the focus on these two people and the consequences of their choices would have been shown differently.
While portraying the young, impulsive lovers sympathetically, Shakespeare also portrays in his characters the dangers of their impulsiveness. Romeo and Juliet is a tragedy, and the tragic ends of the "star-crossed lovers" comes from fate, but it is a fate into which the impetuous lovers have entered of their own accord. Love at first sight is a valid concept, but it is also a most dangerous one.
When you say "valid concept" are you asking if Shakespeare thought it was a good thing, an appropriate thing? That's how I understand this question and I think he did.
If he had not seen it as an appropriate thing, I do not think he would have made Romeo and Juliet be such sympathetic characters. But he did -- he made us really like the two of them and want them to succeed. Because they are defined by their love at first sight, and because Shakespeare makes us like them, I am convinced that he thought love at first sight was a good thing, or at least an acceptable thing for young people to feel.
The young lovers in Shakespeare's Romeo and Juliet would seem to indicate Shakespeare's belief in love at first sight. They do, in fact, meet one time before the romance blossoms.
In fact, the entire play takes only five entire days for Romeo and Juliet to meet, fall in love, marry, consummate the marriage, and die tragically. While they are up against the family feud that rages between their two families, remember also that they never stood a chance of surviving their deep and instant love: they are star-crossed lovers, and so the "gods," have ordained their fates before they were born.
From forth the fatal loins of these two foes,
A pair of star-crossed lovers take their life... (Prologue, 5-6)
This give rise to a great discussion regarding whether one believes in predestination: that their lives were mapped out before they were born, or whether man may choose his own fate. If you have read A Midsummer Night's Dream, also by the Bard, you will recall that the love between Demetrius and Helena is created and fulfilled by the wishes of the fairy kingdom, specifically Oberon—for at the beginning, Demetrius spurns Helena's adoration of him.
Whether Shakespeare believed it, or provided it to appeal to his audiences or both, is not clear, but he presents it unarguably between the young lovers of Romeo and Juliet, a love that has become iconic as one devout and true.
Despite their impulsiveness and, in Romeo's case, fickleness, it is obvious Shakespeare portrays two people who are absolutely in agreement about love at first sight. The main theme of the play, however, is not as concerned with whether love at first sight exists as it is with exploring the obstacles that stand in the way of love. Romeo and Juliet are kept apart because of family feuds, long-time hatreds fueled by pride and stubbornness. Their love is presented as an idealistic balm to this hatred, to the extent that it is implied at play's end that their love may manage to heal the rift between the Capulets and Montagues.