How can I argue that Romeo and Juliet were in love and not just infatuated?

It is difficult to prove that Romeo and Juliet were in love and not merely infatuated. Shakespeare's play takes place over the course of 5 days, from Sunday to Thursday, which is indicative of an infatuation rather than a substantive love. Shakespeare most likely used Arthur Brooke's poem The Tragicall Historye of Romeus and Juliet, which was written in 1562, as the basis for his play. In that work, the story of Romeo and Juliet takes place over the course of nine months. That very time frame makes it easier to point to love rather than an infatuation. Here is a link to Arthur Brooke's work: Arthur Brooke's Poem.

There are some lines that could be used to argue the point that their romance is rooted in something deeper than just physical attraction. When Romeo confesses his love for Rosaline to Benvolio in Act I scene I, he talks only about Rosaline's physical beauty, and the fact that she has made a vow to remain chaste. He says: 

"She'll not be hit with Cupid's arrow. She hath Dian's wit; And, in strong proof of chastity well armed, from love's weak childish bow she lives unharm'd. She will not stay the siege of loving terms, nor bide the encounter of assailing eyes, nor ope her lap to saint-seducing gold." 

He goes on to lament that Rosaline has chosen a life of chastity, and because of this, her beauty will never be passed on to future generations. Her decision to not have a physical relationship with anyone, Romeo claims, has left him alive, but dead. His feelings certainly carry the intensity of an infatuation, but no substance that could be considered love.   

We don't know how long he knew Rosaline from the text, but it's clear she's rejected his advances. The fact that Romeo meets Juliet while still pining for Rosaline certainly doesn't help further the argument that he feels love rather than infatuation. However, we do see how he treats Juliet, with much respect for her honor and something close to piety, and from the exchange in Act I scene 5, an argument could be made that he felt something stronger than mere infatuation.  

In the prologue to Act II, it says:  

"Now Romeo is beloved and loves again,

Alike bewitchèd by the charm of looks,

But to his foe supposed he must complain,
And she steal love’s sweet bait from fearful hooks.

Being held a foe, he may not have access

To breathe such vows as lovers use to swear.

And she as much in love, her means much less

To meet her new beloved anywhere.

But passion lends them power, time means, to meet,

Tempering extremities with extreme sweet." 

This could be used as evidence of the strength of Romeo's feelings, that he is willing to risk his life to woo the daughter of his sworn enemy. He also agrees to marry Juliet, risking his life and future, which suggests that this feeling might go beyond physical attraction. It could also be argued that his willingness to marry Juliet shows concern for her honor, which is an argument for love rather than infatuation. Infatuation seeks to get something from another. An example of this is when Romeo sought a physical relationship with Rosaline. Love seeks to give to another, and Romeo is giving Juliet her wish, as well as her dignity by agreeing to marry her.