Dear Kevin, 

Your question is quite exhaustive and touches on a point that is central to understanding the nature of the priesthood. The promise of remaining celibate that a priest makes is not inherent to the priesthood, but it is the way of life that the Church in its experience over the centuries has discovered is the most fitting human response to the mystery of the priesthood. 

Basically what you ask, and answer, is: is there any way round the promise of celibacy? Could you possibly become a priest and be married at the same time? Then you ask if God is not contradicting himself when he tells our first parents to increase and multiply, yet tells his priests not to. Or would it be OK to opt for married deaconate instead of the priesthood? 

Remember where you are coming from, Kevin. You already have a girl friend and you care for her very much. So celibacy is not an abstraction for you, it would mean giving up a special person you already know, and a way of life that is good and attractive, and a distinct possibility for you. Nevertheless, the impression that God may be calling you to a priest has been there for some time, to the degree that you are pretty sure that you have a vocation. 

You are right to want to know the reasons for celibacy. There is much at stake. 

Remember the story Jesus told about the man who collected pearls? He had a fine collection but one day saw a remarkable pearl that was so beautiful and valuable that it was worth the sum of all he had already collected. He immediately went out and sold all his magnificent collection so as to be able to buy that one special pearl. It would have been dumb to do so if the pearl weren't so great. It's the same with the priesthood. Unless you really know the value of it you will not see why you should give up all the other things that look so good to you. 

Now, only our faith can help us understand what the priesthood is, and its real value. Only faith can tell us how much Jesus loved us, and give us the desire to love him back as much. Only faith can give us trust, and start out on a journey that seems too long and too harsh if we don't know who is going to help us along the way. 

What do I mean by faith? An abstract idea? No, it is knowing what Christ did for us, and what he continues to do for us. It is knowing that in the Mass you renew the sacrifice he offered on the Cross so that all those present can share in its graces; it is knowing that he is present in the Eucharist for us, to be with us. It is to know that Christ takes away our sins in Reconciliation. And it is to know that he does all this through the hands of his priest. Love is to give yourself to others, to be always at their service; to want to be available to help, heal them from their sins, give them encouragement and hope, bear their burdens; love is to spend your life not centered on yourself but on someone else. Celibacy makes us able to do this in the extreme. 

When you become a priest Christ takes hold of you in a special way. He has you do his work, but that is never enough. Priesthood is not a career or a job - he draws you ever more to be like him. The priest needs to become identified with Christ, to think, feel and live like he did. To have his same standards and motivations. To become like him. And Jesus never married. By choice. So the best way for a priest is Christ's way. By choice. 

God wants us to "increase and multiply", but for a priest it is going to be in a spiritual way. A priest is called to give life to souls, to care for them, educate them, protect them, challenge them, at times correct them, give them example, give his life for them. Just as Jesus did. When St Paul described his relationship with the Christians in the cities where he preached the Gospel he called them his "children" that he "brought into life." That is why we call our priests Father. 

Also, one of the major truths that our faith teaches us and we should base our lives and choices on is that there is an afterlife, that the soul is worth more than the body because it lasts forever. Celibacy in a priest shows he really believes that and has staked his life on it. 

I have only skimmed the surface here, Kevin. You will find more in the encyclical letter written by Pope Paul VI called, "On Priestly Celibacy" (24 June 1967). You will be able to find it at the Vatican website and many Catholic websites will give you the link. You can also look up Pope John Paul's yearly letters to priests, written for every Holy Thursday. You will find them in the same websites and they will tell you a lot about the priesthood. 

Be sure of my prayers.