No it's not illegal to open your spouse's mail.
That is as long as your spouse's mail is being delivered to the same address as you then you have the legal right to open mail in your spouse's name.
Also because you're married then you can legally open their mail because it's not seen as just the spouse's mail when you're married and living together or getting the mail at your address.
You can also open your child's or teenagers or adult child's mail if it's being delivered to the same address.
It's only illegal to open someone's mail if the mail is going to a different address other than your address and the person is not related to you.
For example if you go to someone else's mailbox and then get their mail then that would be stealing the mail and then if and when you open that mail then it becomes illegal and likely a felony.