If you file married on your taxes but are not actually married do you have to get a divorce before a marriage?

My husband filed married on his income tax with his ex for at least 3 years that I have seen but they were never actually married and he claimed her children as his but she did not use his last name or anything. Did they need to get divorced before me and him got married? They have had some papers come in from the IRS for money they owed jointly because of filing married and I am just worried that our marriage may not be legal because of this because when I was going to file married with my ex who I was having a child with the tax preparer told us not to unless we wanted to have to get a divorce if we ever decided to marry someone else so we did not file together. If anyone could answer it would be greatly appreciated if not I may have to call the IRS.