Most of the time, when you hear people talk about commingled assets, they are referring to bank accounts and financial assets. For instance, one spouse may have $50,000 in a bank account when the two get married, but they spend the next five years putting both of their paychecks into that account and using it for joint expenses. That commingles the assets and means the entire account is now marital property, even though the spouse who opened it initially may well remember having the $50,000 to start.
It is important to note that you can do the same thing with your home.
For example, maybe you bought a house when you were in college. You lived in it with your roommates, who paid you rent. After you graduated and got a job, you lived in it by yourself. All told, you paid the mortgage for 10 years on your own.
When you got married, your spouse moved in with you. You decided to use your joint bank account to pay the mortgage. This means you used your income and your spouse's income. This "commingles" the house and it now counts as marital property. Even though you owned it first, if you get divorced, your spouse may have a claim.
Married couples naturally share money in most cases, as marriage is a partnership. It is not wrong to do this, per se, but you must think of the big picture and consider the overall ramifications of sharing assets with someone else. It may have a large impact if you decide to get divorced.