The first thing to note about first impressions of Juliet and Romeo in William Shakespeare's Romeo and Juliet is that they will vary considerably. A professor who has been teaching Shakespeare for twenty or thirty years will have a very different perspective on these characters than most students first encountering the play.
That being said, I find them both rather silly, self-centered, and impetuous teenagers. Romeo's infatuation with Juliet at first sight does not strike me as romantic, but rather as a sign that he is not yet capable of forming lasting relationships. As the play opens, he is pining for Rosaline. A few hours later, after seeing, but not even talking with Juliet, he says:
Did my heart love till now? forswear it, sight!
For I ne'er saw true beauty till this night.
If he is that fickle, and that susceptible to simply seeing a pretty face, the likelihood that he would have been a faithful husband strikes me as fairly low.
As for Juliet, she seems somewhat more thoughtful than Romeo, but still impatient and lacking good judgment and consideration for her family.