An 18-year-old American is facing felony charges over claims that she had sexual contact with her underage girlfriend, prompting gay rights advocates to say she is being unfairly targeted for a common high-school romance because she is gay.

The criminal case against Kaitlyn Hunt is unusual because it involves two females, not an older male and a younger female. But advocates say older high schoolers dating their younger counterparts is an innocuous, everyday occurrence that is not prosecuted – regardless of sexual orientation – and not a crime on a par with predatory sex offences.

Hunt played on the basketball team with her younger girlfriend and shared the same circle of friends in Sebastian, Florida, said Hunt's mother, Kelley Hunt Smith.

The two had a consenting relationship that began soon after Hunt turned 18, and Hunt Smith said she assumed the younger girl's parents knew that. The younger girl was 14 when the relationship began, and is now 15.

Hunt was kicked off the basketball team near the end of last year after the coach learned of the relationship because players were not allowed to date each other, her parents said. Then, in February, she was charged with lewd and lascivious battery on a child aged 12 to 16. The day before she was arrested, police and the younger girl's parents secretly recorded a phone conversation in which the two girls discussed kissing in the school bathroom, said Hunt's father, Steve Hunt.

"It's horrible. For my daughter's sexual preferences, she's getting two felony charges. It could possibly ruin her future," he said.

The alleged victim is identified only by her initials in court documents, and her parents have not been publicly identified.

Prosecutors have offered a plea deal to Hunt that would allow her to avoid registering as a sex offender if she pleads guilty to lesser charges of child abuse. The state attorney Bruce Colton said he would recommend two years of house arrest followed by one year of probation if she took the deal.

If she is found guilty, it is also possible that Hunt could apply not to have to register as a sex offender under a "Romeo and Juliet" law because the girls were no more than four years apart in age, Colton said.

Colton said the victim's family was not pushing for prison but wanted Hunt to be held responsible in some way. However, the Hunt family said they would accept a plea deal only if the charges were dropped to a lower-level misdemeanour.

"One of the reasons this case has gotten people's attention is because it's being publicised as a person being persecuted because she's gay, and that has nothing to do with the case, nothing to do with the law, nothing to do with the sheriff's office filing the charges," Colton said. He said the law was designed to protect younger children from older children who might be more aggressive in starting a relationship.

"The law doesn't make any differentiation. It doesn't matter if it's two girls or two boys, or an older boy and a younger girl or an older girl and a younger boy. Whatever the combination, it doesn't matter."

The American Civil Liberties Union of Florida said Hunt was being criminalised for behaviour that "occurs every day in tens of thousands of high schools across the country, yet those other students are not facing felony convictions … and potential lifelong branding as sex offenders".

An online petition urging that the charges be dropped crashed at one point because it got so much traffic.

In the meantime, Hunt has been attending a different school since her expulsion and will be allowed to walk with her class at graduation in June. Her mother said she was expelled by the school board even though a judge had ruled she could stay.

Sebastian River high school's principal and assistant principal did not immediately respond to emails sent on Tuesday.