The girl said she looked up to Phythian. Both Phythian and Marke were working as martial-arts instructors in Nottinghamshire at the time.
She said Phythian and Marke gave her alcohol and Phythian dared her to perform oral sex on Marke. Phythian and Marke were not married at the time of the abuse, the victim said.
The woman she Marke told her no one would believe her if she went to the authorities about the abuse.
Marke faced additional charges stemming from a second victim coming forward. She said Marke began having sex with her when she was 15.
Phythian and Marke deny all allegations, despite the convictions. Phythian has since deleted her Instagram account and Facebook page.
The couple will be sentenced for their convictions on May 16.