By the very fact that Romeo and Juliet meet, fall in love and are married in the course of three days, it is difficult to believe that they experience deep spiritual love. What they feel immediately is an intensely passionate physical attraction which does develop into an erotic love; this love is certainly more than mere infatuation because they are both willing to defend the other and to die when they cannot be with each other.
When Romeo first sees Juliet in Act I, Scene V, he is immediately enamored with her, struck by Cupid's bow:
O, she doth teach the torches to burn bright!....
Beauty too rich for use, for earth too dear!....
Did my heart love till now? forswear it, sight!
For I ne'er saw true beauty till this night. (1.5.42-51)
Also, that Romeo continues to love Juliet after he learns she is a Capulet demonstrates more depth than mere sexual attraction; he is willing to risk his life to love her. Likewise, Juliet's feelings for Romeo are more than infatuation, for as Romeo stands beneath her balcony in Act II, Romeo hears the fair Juliet bemoan that he is a Montague, but declares that his name is not important.
Deny thy father and refuse thy name.Or, if thou wilt not, be but sworn my love,And I’ll no longer be a Capulet. (2.2.34-36)
My name, dear saint, is hateful to myself,
Because it is an enemy to thee;Had I it written, I would tear the word. (2.2.55-57)
...I'll to my wedding bed;
And death, not Romeo, take my maidenhead! (3.2.139-140)