I believe that if I were in the position of the priest, my motives would have been dictated by money, truthfully. Here's why: Let's say that R & J are paying the priest to marry them (hypothetically). If I'm the priest, it is in my best economic interest to marry them, collect my fee, and allow fate to take its course. If, however, we are speaking strictly on the religious level and our priest is receiving no payment for the ceremony, then, yes, I might have spoken up. If there's nothing to lose but something to gain, that becomes the deciding factor in a very pragmatic sense.
There is a lot of discussion on this issue, actually. Many people say (and I tend to agree) that if the priest had intervened, there might have been a way for the whole scenario to turn out differently. If he had told someone what was going on, Romeo and Juliet might have lived a long and happy life together (or apart, but at least not dead).
But then, of course, it wouldn't be one of the greatest tragedies of all time. Shakespeare included these inconveniences in the plot so that things would work out the way they did.
I think that if I were in the father's position, I would have intervened. I know he was hoping that the marriage would bring the two families together, but when he saw how badly things were going, he should have spoken up. After all, Romeo and Juliet were very young, and I think he placed too much responsibility on their shoulders and left too much to them. As the adult and the spiritual authority, he probably should have been more forward. But like I said, I love the story the way it is; this is just what I would do in that situation.