Your concerns about his kids are legit, if you are sure you do not want them in your life - that's nothing to be ashamed of. Would his ex be in your life if you married him? I would be more concerned with his relationship with his ex and how she fits into the equation.
You can't blame him AT ALL for having a past relationship. How he deals with that past relationship NOW is what matters. If he didn't have his past, his life would have been different and you two probably would have never met. You seem to be idealizing what you see as a perfect life with a perfect husband. 90% of people have screwed up lives in some way. The other 10% lie about it. Gotta take the good with the bad sometimes. Everyone has baggage. In 5 or 10 years you could see a stepson as worth it, even as something positive.
A few things I have learned personally and from friends... the core of the relationship must be between you and him. This means don't stay in a relationship because you love his kids or his dog or his friends or his material possessions. Stay because you love him. On the flip side, don't let a good thing go because of a few complications. Everyone has complications or problems. True love is worth a few complications and sacrifices - how much you can handle is up to you.
Maybe you need to get out and date more, or spend some time apart from him to get some perspective. You can't change him or blame him for his kid - you can only blame yourself for staying in a relationship with complications that YOU could not get past.