I mean you wouldn't HAVE to, you can do anything you want, it's just a matter of whether it's compatible with being married. I see that as the fairest bare-numbers solution for all involved.

If you can get her agree to just you buying a house alone in your name only and she pays utilities or whatever, sure. (Not sure if you'd figured on putting a house in your name and her putting toward the mortgage at all without having long term stake. That's almost like a renter situation, which is weird IMO for marriage.) You may want to check your state laws regarding major purchases made during a marriage vs before, I think my "commonwealth" state might consider it communal property even without the deed formally reflecting that, not sure.

My impression so far here is that she wants to have something to give her kids, and anything that's considered both your and her property going forward will NOT be given to them in any part, which is where I can see it being justified for her to keep the townhouse separate. IF you were to agree to split things between all the kids, would that change her mind? Because if she then sold the townhouse and went 50-50 on a new residence with you, it woudn't be just you funding them. And frankly, by the time it's an issue, you'll likely both be dead anyway / does it matter THAT much?