You haven't answered the question yet...."..tell the readers here how polygamy is illegal...using citations to the 14th Amendment in your answer." Also you can use citations from Obergefell added to your citations from the 14th.
Polygamy is still illegal because the laws still stand
Neither Obergefell or Loving addressed polygamous marriages because neither case was in regards to polygamous marriage.
If you want to challenge the marriage law so you can marry 3 men- you can do what the Lovings and the Obergefell did- you can go to court saying your rights are being violated. You have the right to challenge the law in court- you don't have the right to win.