Birk Billingsley knew the price of his free Naval Academy education.

It required adherence to all the dos and don'ts. Still, last fall, the 22-year-old senior engaged in a don't. He got caught and had to head home to Texas. He would have graduated in six weeks. Instead, he owes the Navy $93,000 for his education.

"I made a big mistake," he says.

After 22 coed years, the academy still struggles to regulate the urges of college-age men and women. Despite the rules -- no sex with a freshman, no sex on campus -- midshipmen regularly fall in love, have sex and sometimes get caught. Four were expelled this year for sexual misconduct or fraternization, pending the Navy's final ruling. Five were expelled last year.

Amid recent news of improper relationships in the military, the academy superintendent, Vice Adm. John R. Ryan, has reminded midshipmen that he won't tolerate fraternization. But academy officials know they're restricting activities that, on any other campus, are commonplace.

Used as an example

Billingsley, who walked out the academy gates yesterday, feels he was expelled as an example -- proof of Ryan's sincerity. Still, he left with a head full of emotions: anger (at himself and the school that no longer wants him), sadness and fear.

"This is something I wanted to do since I was knee-high," he says. "After immersing yourself in a place for four years, you watch it go down the drain in front of your face."

In a letter to the secretary of the Navy, asking that his expulsion be overturned, Billingsley wrote: "I feel as though I have run the race but tripped at the finish line."

Billingsley's expulsion is an example not only of how academy love affairs can dash dreams of a naval career but also of how matters of degree can make the difference between being expelled or not. Some Mids have had sex with freshmen off campus and not been expelled, while others have merely kissed on campus and been told to leave.

Mids who have been expelled say the system seems disparate and unfair.

Last year, Navy quarterback Chris McCoy was accused of having sex off campus with a freshman, or "plebe," who was also accused of having sex with two other Mids on campus. She was expelled. He wasn't.

Last fall, in an incident similar to Billingsley's, senior Colin Gallagher fell for a plebe in his squad. They, too, were accused of fraternizing, but the academy didn't expel them because the couple denied having sex.

A tough call to make

Spokesman Cmdr. Mike Brady calls them "different cases with different offenses. There is no formula or template used to determine outcomes and it is wrong to assume that every case should be treated identically."

While some violations are clear-cut -- drugs bring automatic expulsion -- love and sex are harder to adjudicate, academy officials say.

"The emphasis of the whole system is finding ways to keep people, not finding ways to separate people. To get separated from here is actually pretty hard," says Col. Dutch Schlaich, the deputy commandant, which is akin to an assistant dean of students.

To clarify the rules on who can date whom, who can have sex with whom, Ryan told all 4,000 Mids at the start of the school year that he was "placing a bubble" around each freshman.

"There are certain boundaries you cannot cross," he told them. "It usually begins innocently enough -- on a team trip, with e-mail, -- but it gets bigger, it gets out of control and eventually it can ruin careers, lives and people. Don't let this happen to you."

Innocent beginnings

But Birk Billingsley wasn't thinking about bubbles or matters of degree when he and Kristina Shiroma began flirting one day on the streets of Washington, D.C.

Billingsley first heard Shiroma's name from his former girlfriend, who had met Shiroma in 1997 at a Texas girls camp. The girlfriend told Billingsley, then a junior, that Shiroma was headed to the academy the next year, and he should look her up and offer to help her.

Billingsley was surprised to find Shiroma in his squad last summer, living down the hall from him. During a squad field trip to Washington, he found she had a sense of humor. They talked about their home state of Texas, began e-mailing each other, meeting around campus.

"This was never the case of some firstie [upper classman] out to put another notch in his belt. No one was a victim here," he says. "I think we both knew it wasn't common for a plebe to be dating a firstie. But I don't think we discussed the ramifications."

Through the fall, the relationship blossomed. They sneaked into quiet corners of the dorm and hidden spots around campus to hug and kiss. They met at the home of a local resident who had befriended Billingsley, at a Washington hotel, back home in Texas over Thanksgiving.

Romance revealed

In December, an officer told Billingsley someone had turned him in. After interviews and hearings, he got the word on St. Patrick's Day: You are being "separated" from the academy.

Billingsley says he and Shiroma, who has also been recommended for separation, had sexual relations but no sex. But his superiors said the relationship was unacceptable because Billingsley was Shiroma's supervisor, her squad leader.

"That's why to me it's so problematic," the commandant, Rear Adm. Gary Roughead, told Billingsley during a conduct hearing.

(Midshipman Gallagher was also squad leader to his girlfriend. An academy official says Gallagher was not expelled because the couple did not have sexual relations. Gallagher will graduate next month. The woman resigned last fall.)

Billingsley grew up in the small town of Dublin, Texas, south of Dallas, son of a high school science teacher and a construction worker. He first thought about attending the Naval Academy after writing a report in fourth grade on academy graduate and astronaut Alan B. Shepard Jr., who died last summer. Later, he attended a weeklong "space camp" in Alabama.

Horrific experience

"He's wanted this since he was 10 years old," says Billingsley's mother, Rita Kasper-Cheney, a teacher at Dublin High School. "His experience there has been wonderful -- until now. This has been a horrific experience."

Now, Billingsley is reluctantly applying to Stanford, Georgia Tech or Purdue universities, to finish his aerospace engineering degree.

But first there's the $93,000. An alternative is to serve three years as an enlisted sailor in the Navy. "Neither one is appealing to me," he says.

Billingsley turned in his class ring March 29. That's when the finality of it hit hard.

"That was one of the toughest things I had to do," he says.

His bags had been packed for two weeks, stacked in his nearly empty dorm room. He had stopped attending classes. But Billingsley was waiting yesterday for a final piece of paper to be signed, sealing his departure, so he could walk through the gate and begin the journey home.

"This has been the most humiliating and excruciating experience of my entire life," he says. "To come this far, it's just really difficult."