Don't get pregnant with a man who does not want to marry you. Getting a dog together has zero to do with building a marriage (even one that isn't legalized). It's also no real sign as to how you two would react to and deal with a child. Honest. Not a practice run.
I suggest you chill on this until you are 24 or 25 and find out whether or not the two of you are even still together. You are in a very formative part of your life. However, your mind, scientifically proven, will not emotionally mature until you are at least 25 years of age. And you may very well feel differently about this man when you are 25 and he is 35 and still unwilling to "commit" to you in a meaningful way.
BTW, when you wed, you do NOT take on your spouses previous debts. No matter what name you take or don't take. Previous debts remain the obligation of the person who incurred them. Married couples are held responsible for many debts incurred DURING the course of the marriage though.. and even then, not all of them! If my husband went out tomorrow and bought a home in his own name, on his sole credit, and he passed away.... I might own the the accrued value minus the debt load.. but if I wanted to keep that house, I'd have to qualify for the balance of the loan. I'd not inherit the debt... I'd just not own that which wasn't mine.