My boyfriend still has a joint account with his ex-wife..........?

My boyfriend has 3 children with his ex-wife. We have been dating for 2 years, he has been divorced for 3. They share a bank account "for their children". He says they both put money in it periodically to use for the children. This bothers me because if him and I were to ever get married he has an account with another woman regardless of the reason. He pays monthly child support to her (ALOT) and his kids all have pre-paid college so I know the money is not used for that. Am I wrong for being irked by the fact that they still have a joint account?