Well, I've held off for a while because usually I am in prevention mode and don't have much to say once, well you know. (can't think of good expression without it being tasteless pun) I have not had to bear this burden as a parent, but I have seen plenty of "good" kids make this bad decision. It doesn't help that he doesn't have a father to guide him. Hopefully there are some other Christian men at church who can knock him to his senses (figuratively, of course) and also help him cope with his hormones. This may sound strange but your husband (are you married - can't tell from your icons) can play a pivotal role in all of this. No better person than his girlfriend's father to take him under the wing.
Let's see - some practical stuff.
Let's not lose sight of how nearly impossible it is for teens these days. Ignoring their real life struggles with acceptance, self image, and the sheer force of hormones does no good. It is good to acknowledge that you understand why they made the decision they made, even though it is an unhealthy one.
I also recommend "The Bride Wore White", as well as "Every Young Man's Battle" and "Every Young Woman's Battle". They need to know the reward for waiting and also know that the battle against culture and peer pressure can be won. One further book that I think they might enjoy is "Sex God" by Rob Bell. It really outlines the prupose for sex as God designed it, and helps everyone, but especially young ladies, understand what a valuable treasure they and their purity are.
How serious are the two of them? Are they really at a stage in life where marraige is a possibility? If not, I would suggest that they "cool down". They should work on the relationship part of their relationship. Staying out of environments that can lead to "burning with passion" is essential and will help them focus on the emotional side of their relationship as well as their growth in Christ. I would lay it out this way -
"Both of you know now how easy it is to fall into satan's trap. You know how powerless you can be when passions flare. You both agree you don't want it to happen again. The key to this is to keep yourselves in safe environments. We are on your side in this, and so understand that if we deny your request to go somewhere or do some thing, it is only because we know the potential dangers that those places and activities hold. Hopefully, you will be able to monitor yourselves, and won't need our oversight. But we will not waver in our commitment to help the two of you remain pure. Sometimes, you will think we are stupid and don't understand. But you can't see through the fog of love what we can see and you don't have the experience to know what pitfalls are out there. So you will have to trust us. Sometimes you will think we are trying to break up your relationship. Know that we aren't. We are, in fact, trying to help your relationship grow. But we have the experience to know the safest path to take. So you will have to trust us. And sometimes you will think we simply don't care. Nothing could be further from the truth. We have only your best interets in mind. We are all on the same team working toward the same goal. But the reality is, we have been through this battle. We simply know best and you will just have to trust us. Know this one thing - romantic dating has only one purpose and that is to find a mate. If you do not feel ready or are not in a season of life where finding a mate is on your radar, then you need to back away from romance. On the other hand, if you feel ready and do want to pursue marriage, then buckle your seat belt because you are acknowledging that you no longer want to be treated like a child. Children do foolish things and make foolish mistakes. Being an adult is serious business and invokes serious scrutiny from the other serious adults in your life. You want to be treated like adults, then you need to be accountable like adults. You want to be treated like adults then you need to show responsible behavior like adults. We will not hold any punches or sugar coat things for you. If this is your desire, then you need to cease thinking, acting, AND REASONING like children. Are you up to that challenge?"
OK - those are just random thoughts but you get the idea.
On Birth Control - it's a tossup to me. I think putting her on it basically conceeds the fight. But an unwanted preganancy just compounds matters. The sense I get is that the two of them desire to remain pure from here out and so it seems unnecessary.
One last thing that we told our girls. It was kind of the "as long as you are under my roof..." deal. They don't have to like your rules to obey them and obedience is mandatory. We told our girls simply - "you can have an adolescence that is relatively free or one that is tantamount to prison. It's your choice. Act trustworthy and you will be trusted with all the freedom and privacy that entails. Betray our trust and you will be distrusted and scrutinized mercilessly. With every action you take, consider this reality." To prove the point, we occassionally followed our girls, crashed parties they were at, and demanded strict itineraries and proof they followed them. It didn't take them long to know we were quite serious. They CHOSE to do the right thing, not because they are such wonderful compliant girls but because the alternative was taking too big a chance that they would lose their freedom. In turn, they actually had a great deal of freedom AND learned to stand against the crowd and have self confidence. At the same time, they saw their friends face one life crisis after another. In the long run, they realized that our rules were not there to ruin their fun, but were actually sensible and helped ensure they could have fun while remaining safe and healthy.