The question to ask yourself is whether you care enough about your friend to let him go. He has made the difficult decision to marry and be a dad when the circumstances aren’t the best. He deserves support for that decision. You’re right that it’s not appropriate to tell him how you feel. It’s also not healthy for either of you for you to be talking negatively about the woman he has decided to be with. He and the mother of his child would probably benefit from some counseling to help them cope with this unexpected change in their lives. Their relationship deserves a chance. Their baby deserves to have two parents if it’s at all possible.
Where that leaves you is grieving the “what if guy” in your life. It’s a real grief. If things were different, something more than friendship might have developed between you two but you’ll never know. It’s important that you give yourself room to grieve. Then it’s equally important to do whatever you need to do to move on. Dating others won’t work as long as you are still holding out hope for your “what if guy.” Your heart isn’t in it. The guys you date sense that you just aren’t into them.
Find a way to take some distance from your guy friend. Wish him all the luck in the world. Then start putting yourself into activities that genuinely interest you where you can start getting to know other guys. Working together is a perfect way to get to know people well without jumping right into dating. You’re not ready for that. But you do need to get busy enough so that you’re not tempted to isolate and get depressed. The antidote to loneliness is to be with other people.
I understand this is terribly hard. I do sympathize. But I also think real friendship sometimes calls for real sacrifice. I hope you will do your grief work and then give yourself the chance to find the guy who is out there waiting for you.
I wish you well.