please read all the posts because you are missing some of the earlier discussion. There are other exceptions beyond a pre-nup that trump community property per my lawyer.
I think we all know what the OP should or should not do, so leaving that aside and focusing on the general or theoretical situation at hand:
For all practical purposes, it would be virtually impossible for someone in the OP's situation to keep his separate property (as declared by him), separate,
even if he could trace the down payment to clearly separate $$ that predated the marriage. As Arebelspy points out, this is a man who provides the sole income for the family. All continuing mortgage and maintenance payments will come from marital assets (his income), and thus will immediately be commingled. And, the family will be living together in the marital home.
So, assuming the soon-to-be-ex wife presses her claim to half the home's value (since this is the only situation we are interested in, it's a non-issue if she walks away on her own) then, as pointed out, the burden will be on the husband to show that it was his sole, separate property that completely predated the marriage that made the down payment, and that will definitely be a high burden. Even if that was established, the down payment will be immediately commingled with marital property in the form of the very first mortgage payment. Once it's commingled, that just about puts the nail in the coffin, so to speak.
It's just a fact that established law and precedent considers property acquired during a marriage to be joint property, absent evidence to the contrary. There are no doubt various state laws and exceptions, but generally it's not going to matter how it was acquired. A court will not presume the marital home was purchased with the intent of excluding the wife from half the value, absent a signed agreement from her specifically stating that to be the case. This will be particularly true for a family with a single wage earner -- his income is her income, and everything bought with it is half hers.
Now, in the case of the husband purchasing the home with his own separate, traceable, pre-marriage $$, and a month later the marriage dissolves, he might be able to get his down payment back just to himself. But after any significant passage of time, forget it. The court is not going to entertain teasing apart down payments, ongoing payments, appreciating value, repairs, improvements, sweat equity, etc. -- it's going to be split, period, particularly on a marital home which is, by definition, marital and not separate.
Furthermore, depending where you live, the court may heavily frown on attempts to disinvest a non-working, dependent spouse from shared assets like a marital home, so if there are other issues being argued (like child custody, pensions, other assets), it could backfire attempting to press such a claim (besides having little or no chance of success).
I'll even go a step further: I think even if a husband in this situation got his wife to sign a post-nuptial agreement forgoing any claim to the marital home value, she would have an equal or better chance of having a court toss that post-nuptial agreement and giving her half the home's value, then his chances of tracing his own, separate down payment and getting it back without a post-nuptial agreement. It's jut too lopsided against the wife.