You say he's 46, a workaholic, and successful financially. That he wanted to be a financial workaholic success and so he's put everything into this, assuming approximately 25 years, and as such, did not want to put the time into having a wife / having children
Now he is reached the peak of his work-life (most likely) and standing at the top of the peak looking down, he's thinking, I am fit, I am active, I am healthy, I've achieved most of my work goals, and I and financially secure. Now that I have this, I can start a family.
This makes sense to me. Although lots of younger/financially insecure people start families, and struggle through and do well enough, men have the unfair advantage of being able to put off family/children almost as long as they'd like to as long as they end up with a younger/fertile mate.
You say you're worrying because you feel like he's just marrying the first womb that comes along...
But you didn't mention how long he was your fiance. Did you guys just meet?
Because if you've known him for years, and you've had a good solid relationship for years, then maybe this isn't such a terrible thing this is just the natural progression of your relationship.
But if you just met him say less than a year ago, and now all of a sudden he's talking about marriage and children, what do you think? You say you're 31, child free, and enjoy having your personal time and time away with friends. That, if you become a mother, you already know you're going to be doing the majority of the child care because your workaholic husband is still probably going to be a workaholic and more than likely financially providing for the family.
I don't think the age difference is a gigantic issue unlike others here. While it is true that having 15 years between you could mean that his energy level when the kids are 5 or 10 years old isn't going to be the same as your energy level, you honestly don't know that for sure. There's plenty of partners who are younger who have physical or psychological issues that drain their energy and motivation, and plenty of other older people who have tons of stamina.
You just have to have a discussion about family planning. Everyone does regardless of their age, regardless of their career, all partners who are of childbearing age need to have this discussion before "something happens" And the discussion is a moot point because the decisions been made because you've gotten pregnant. And then, it'll either all work out the way that it should, or they'll be resentment and disappointment and inevitable divorce because you guys weren't on the same page from the beginning regarding family and division of duties.