Romeo and Juliet, it is believed, is likely a variation of the Persian tale Layla and Majnun rewritten by Shakespeare for a Western audience. To bridge the culture gap that then existed, Shakespeare had to make some significant changes.
The most pronounced and discussed of these is consummation (or lack thereof). In the Persian tale, the love that Layla and Majnun have is "virgin love", that is, never consummated by sexual intercourse. Romeo and Juliet do famously marry and enter into those relations. The reason for that particular variation is that the two cultures viewed (view?) sex and marriage in entirely different lights.
There is much glorification in Romeo and Juliet of the giving and the taking of the maidenhead. Much of the play seems preoccupied with it. The play opens with two bawdy young men bragging that they will kill the men and rape the women belonging to their enemy's household. Juliet's nurse, her mother, herself, even Friar Laurence speak of it with a candor that would certainly not have amused Queen Victoria. For a modern audience, this can be uncomfortable when they realise that Romeo and Juliet themselves are but 14 and 13 years old, respectively. In Shakespeare's time, that was the common marriage age, but to a modern audience they are only children.
Consider this passage spoken by Juliet on her wedding night while waiting for consummation:
"Lovers can see to do their amorous rites
By their own beauties, or, if love be blind,
It best agrees with night. Come, civil night,
Thou sober-suited matron all in black,
And learn me how to lose a winning match
Played for a pair of stainless maidenhoods.
Hood my unmanned blood, bating in my cheeks,
With thy black mantle till strange love grow bold,
Think true love acted simple modesty. ...
O, I have bought the mansion of a love
But not possessed it, and, though I am sold,
Not yet enjoyed. So tedious is this day
As is the night before some festival
To an impatient child that hath new robes
And may not wear them."
To the culture of Shakespeare's Europe and time (and indeed today), love could not exist without sex, though sex could exist without love. An unconsummated love was no love at all. But to the Eastern culture that birthed the story which inspired Shakespeare, the opposite was true. Love in its purest, most divine form was always a virgin love, never consummated, for consummation made it earthly and of men.
Both stories end in the deaths of the lovers, but here too is a major variation. To Shakespeare's England it was nobler to die for love than to live without it. Maybe this romanticism was born of the fact that, at that time, almost no one actually married for love. And that is the choice both Romeo and Juliet make (or are fated to, depending on how you choose to interpret the chorus' prologue to the play). But to Persian culture, to die for it was an easy out and no proof of faithful, eternal love. (Remember, too, that almost no one married for love there either.) It was recognised that it much more difficult to live the lie of being one person's lover while in actuality being in love with someone else whom one was bound by honour and duty to be always sundered from. Hence, this is what was glorified there. To experience such a state and be faithful in one's heart was to be closer to the love of the divine than any other human on earth. Thus, Layla is married to another and eventually dies of a broken heart while Majnun roams the wilderness spouting poetry for his Layla and dying years later near her grave. In this way, Majnun attains to the status of a lesser god, for only a god could possess so pure a love.
Regardless of the variations enforced by cultural receipt, the stories are both enduring examples of the power of love.