Opening someone else's mail is only illegal if you open their mail without their permission or you're not listed on the mail along with the person.
Usually if you live at the same address as your husband and you get your husbands mail and open it then it would not be illegal.
Now if your husband lived at a different address and you went to his address and opened up his mail then it could be illegal.
But if you get your husbands mail at the same address then there's nothing illegal about opening your husbands mail.
I open my husbands mail all the time even though his name is only on the mail and besides he wouldn't turn me in anyway.