The mortgage lists the equity, joint implies 50-50 unless you were to specify otherwise.

There is zero difference if you are married, best friends, siblings, co habitating, two people who had never met before etc. In all likelyhood a divorce would change the equity, not keep it 50-50.

But if you die without a will there will be different heirs for your part of the property.

So the bank really should be asking for a copy of the will rather than a certificate of marriage.