I don’t see the limitations on how the Universe came to be as having anything to do with my understanding of naturalism.
The way I see it, Naturalism would say, “What is, is.” It wouldn’t concern itself with how it came to be.
Either the Universe popped into existence all at one time, as described in the Big Bang model, or it has been there all along, with matter popping into existence from nothingness at random intervals in empty space as in the discredited Steady State model.
Or maybe it is cyclic, expanding and contracting. Aside from being something to talk about and think about, what practical relevance does it have to how one chooses to live his life? One can be evil for any reason, or good for none at all.
Some of us ask these questions because we seek understanding. Others seek guidance. They’re probably on equal footing with getting a satisfactory answer.
Just about every philosophy says "What is, is". If that is all you mean by Naturalism than its is certainly easy to defend, but is sadly void of any content.
My understanding of Naturalism is one where it is more ambitious. It includes the assumption that no events may have super natural causes. For example if I claim I recovered from a sickness because Jesus healed me, it would say, no, you must have gotten healed from natural causes. And if I claimed something that excluded natural causes such as my leg suddenly grew back in an instant then it would say, that I was either delusional or lying, or possibly nature works quite a bit differently than we thought. However it would never allow for a super natural agent such as God to have decided to heal my leg.
Naturalism presumes that there is no God, or other super natural agents that transcend nature.