I have known several couples who did marry to have sex because they did think it was important to wait until marriage. THe problem seemed to be that they were focused on the warm, loving feelings that they had for each other and thought they could only manage them by getting married. They didn't think that there could be another way to stay connected without the sex.
These couples who marred too early also did not have any idea what was required to have a successful marriage. My daughter, who was pregnant at 19 and living with her boyfriend thought that they could live with me. She got a job but her boyfriend didn't. I told them from the start that they were now a family and they needed to find a way to build this family without remaining children with me. They are no longer together.
Most kids, even when they think about being on their own, are unaware of the actual cost in money, time, and emotion to maintain a marriage. Has your son had an income, even just an allowance, for which he was responsible to budget. Does he save money towards future purchases or does he spend it as he gets it.
Have the two of them decided together what kind of a life they want together? Do they agree on how to manage their money? Do they know how much food and rent costs? Do they have similar goals for their futures. ie: do they both want to go to college or a trade school. How will they complete their education if they are still unskilled in doing work that pays a living wage?
There are many questions that have to be discussed and many decisions that have to be made so that a couple can have a good marriage. I suggest that you find a Preparation for Marriage Class and suggest that they go. I think it would even be reasonable to say that you are very uncomfortable with the idea of marriage this young but you'd be willing to think about it with them once they take that class.
I don't know how young people keep celibate in these days. In my day we did not spend time alone. Parents might be in the other room but they were present. We dated mostly in groups even at 17. Going to a movie together works if their time is planned for after the movie. I think I'm saying that to spend time together when one is sexually attracted to the other their time needs to be structured in a way that the temptation is limited. I still do that for myself when I don't want to get sexually involved. I'm older and experienced now so I can spend time alone with the man but I know the signs that indicate that I need to leave or change the activity. I also have much more will power than a teen.
If your son is have intercourse or even heavy petting I'd encourage the couple to be on birth control. It's been my experience that once a teen begins a serious sexual relationship they usually can't stop. Being a teen parent is a difficult life and it is not fair to the teens and especially not fair to the baby who did not get a choice in being born.
My daughter had her first baby at 20. She's now 28 and has 2 children. She frequently says, "why didn't I listen to you, Mom! This is really difficult!" I do help her with child care and with money at times. She feels inadequate because I have to do that. She has a good job and is learning about herself and is beginning to know who she wants to be. Often the children do not get enough attention from her. Both their mother and they are learning and maturing but not together. Often their two stages of maturing are at odds with each other.
When my daughter is tired, sick, and stressed she has no energy for providing the emotional needs of her children.
If you know of someone who has a baby or toddler it might help to ask them if your son and/or his girlfriend could stay with them for a day and a night or perhaps the weekend. Give the teen responsibility for the baby. The parents are there to see that their baby has his needs met and to teach how to do such things as change a diaper, know when the baby's hungry, how to soothe them when they're tired, etc.
Does your son have responsibility for chores at home. If not, assign him to fix one meal a day or perhaps just 2 or 3/week. Have him do his own laundry. (my daughter is now with a 28 yo man whose mother is still doing his laundry.)Put him in charge of washing dishes, vacuuming and sweeping. If he isn't working insist that he do so and then have him pay "rent." If he has a car does he pay all the expenses for it? If not it's time for him to do so. Do all of this good naturedly!!!!!!!!Let him know that by asking him to do these things you are supporting him so that he can make an informed decision about marriage. Tell him if he can manage the responsibilities of being an adult then marriage is OK.
Avoid saying anything negative about marriage. Discuss what happens in a marriage. Never argue or try to convince him of anything. Just set it up and encourage him to take on more responsibility. Be a good listener. If you're feeling anxious and are unable to just ask questions and listen to his views then don't say anything.
It has taken me 10 years to be able to consistently listen and only give my opinion when my daughter asks for it. Since she's an adult now doing so seems obvious. But if I'd done more listen and less "teaching" the two of us would have had a better relationship. It is possible, even, that she wouldn't have gotten pregnant so early. Many factors were a part of their decision. I was just one factor and I believe that I was not successful in showing her real life. I protected her too much. I thought that if I just made a rule, told her to not do something along with discussing the reasons for this rule she would learn. Seventeen is too full of hormones, too much in a hurry to be independant and show Mom and Dad that they can do it because those reasons don't apply to them. They're different. etc.
This is why teaching needs to less talk and more natural consequences. Consistency is still the most difficult behavior for me. Finding a way to listen without getting so emotionally involved is my success.
I wish you well, You are in one of the most difficult roles you will have. Parenting a teen is more difficult than parenting at any other age.
If your son learns thru reading there are many good books, both fiction and nonfiction, on this subject. Reading could give you a look at a bigger picture which could help.