It comes down to something called inception of title. When determining whether marital property is community or separate property, the court would look at when the property was acquired, the inception of title. If the property is acquired before marriage, then it is generally going to be characterized as separate property. If acquired during the marriage, the property will generally be characterized as community property, unless it was acquired by gift, devise, or descent.
Although all property does not become community property once you are married, there will be a community property presumption. This means that any property you have during the marriage will be presumed to be community property unless and until it is proven otherwise by clear and convincing evidence. Deed transfer dates and other documentation could be used in the case of the houses.
If you were to make some of the mortgage payments on your spouse's separate property home, then your separate property estate would have a claim for reimbursement if you were ever to divorce.