While Friar Laurence's act of marrying Romeo and Juliet without parental consent did not break any ordinances of the Catholic Church, his decision to marry the couple may not have been the wisest decision. At this time period women could legally marry without parental consent at the age of 12, while men could marry at 14. We know that Juliet is exactly 12 years old, while Romeo is older. Hence, Friar Laurence did nothing underhanded that broke either the laws of the church or the state by marrying them. However, since his motive in agreeing to marry them was that he believed that the marriage would put an end to the feud, as we see in the lines, "This alliance may so happy prove, / To turn your households' rancour to pure love" (II.iii.94-95), he should have been wise enough to see that the families were not yet ready and that the secrecy of the union might backfire.
Friar Laurence did have a possible alternative. We know from early on in the play that Lord Capulet is beginning to feel willing to try and maintain peace; he is also feeling the pressure of the law to start maintaining peace, as we see in his lines to Paris:
But Montague is bound as well as I,
In penalty alike; and 'tis not hard, I think,
For men so old as we to keep the peace. (I.ii.1-3)
From these lines we see that not only is Capulet feeling pressured by the law, he is beginning to feel guilty about his actions. He believes that men of his and Montague's age should be able to behave better.
Not only do we know that Capulet is beginning to feel guilty about his actions, we also learn that he actually feels a great deal of respect and admiration for Romeo. When he and Tybalt spy Romeo at his feast, he prevents Tybalt from attacking Romeo, saying Romeo's bearing, or behavior is that of a nobleman's, as we see in the line, "He bears him like a portly gentleman" (I.v.69). The word "portly" in this instance can be translated as "stately," or "dignified," like a nobleman (Collins English Dictionary). Capulet goes on further to argue, "Verona brags of him / To be a virtuous and well-govern'd youth" (70-71). Hence we see that Capulet is very willing to think well of Romeo. If this is the case, then Friar Laurence's alternative would have been to postpone the marriage until Romeo could ask Lord Capulet for his daughter's hand in marriage. This probably would not solve the entire problem, but at least the couple's intentions would have been publicly made known, thereby creating an excuse for at least a little civility between the heads of the two families.
Act 2, Scene 3, by the way.