Former Castle Hills School student Kyran Lee was known as Fiona Manson when she duped the victim into a relationship, Lincoln Crown Court was told.
The victim was contacted by someone she believed to be a “tanned, muscular young man” called Joey G-Star Crislow.
After talking on Facebook and by phone and text they ended up in bed together.
Jonathan Straw, prosecuting, said: “Lee first met the victim through the internet using the name Joey G-Star Crislow.
“They contacted each other through Facebook, text and phone calls.
“Lee sent photographs of himself suggesting himself to be a tall, tanned muscular young man.
“He said he had a daughter. The victim was a single parent.
“Lee sent her presents including an engagement ring.
“The victim throughout this time said she wanted to meet him, but Lee made excuses.
“When they met, she realised he didn’t look like his photograph.
“They spent time together and shared a bed. Lee wore a bodysuit and the lights were off.”
The jury was told it was only by accident from a chance conversation that the victim discovered “Joey”, as she knew him, was actually Fiona Manson.
As a result, a complaint was made to police and Lee was arrested and charged.
Lee, aged 25, of Baines Road, Gainsborough, admitted a charge of assault by penetration between September 10, 2012, and October 15, 2012.
Details of the offence can only now be published after a Contempt of Court Act order preventing publication of the offence was lifted.
The offence was revealed during the trial of a second case against Lee, who was accused of sexually assaulting another woman.
A jury heard how Lee posed as a man called Daniel Blades when he befriended the teenage single mother.
Lee denied two charges of sexual assault and was cleared by the jury.
Lee together with Kayleigh Hall, 28, of Ropery Road, Gainsborough, denied a further charge of witness intimidation over alleged threats made to the complainant in the case which went to trial, and both were found not guilty.
During the hearing, Lee admitted using a male identity in relationships with both of the women.
However, Lee said sex did not take place with the second woman.
Lee told the jury he was born female, but from the age of 11 began to question his sexual identity.
In evidence during the trial, he spoke about the woman he has admitted sexually assaulting, saying: “I spoke to her as someone else. Joey.
“I wanted to meet her as a man because I loved her.
“It was only after she told me that the relationship couldn’t continue unless I did have a sexual relationship with her.
“She wanted me to do things I didn’t want to.
“She said that the relationship couldn’t carry on unless I did.”
Lee was granted bail and is to be sentenced later this year for the assault by penetration charge he admitted.