Any adult may petition a court for an adoption under this subtitle. A petitioner may petition for adoption of a child 180 days or more after a child placement agency places the child with the petitioner. If a petitioner under this section is married, the petitioner’s spouse shall join in the petition unless the spouse: is separated from the petitioner under a circumstance that gives the petitioner a ground for annulment or divorce; or is not competent to join in the petition. If the marital status of a petitioner changes before entry of a final order, the petitioner shall amend the petition accordingly.