Last Het Romance

Enrique: If it didn't work with you, it wouldn't work with anyone! I never really knew who I was. You removed all my doubts. I swear, it's not you. I think you're pretty, attractive, sweet; it's me, I promise you, it's me.

Marjane (narrating): Nevertheless, somewhere inside, I was reassured. It was easier to accept that he was gay than that he had preference for Ingrid or he found me ugly.

—Persepolis

This girl is responsible for a guy in a Transparent Closet realizing he's gay, and inversely this could be a guy responsible for a Closeted Lesbian realizing she's gay. The situation is usually that after their first kiss (or sexual encounter in some cases) he realizes that he doesn't feel the right kind of chemistry with her, and outs himself to her and breaks the relationship. Usually this requires that the ex-boyfriend profusely reassure her that "It's Not You, It's Me" and that she'll eventually find someone who will return her feelings. On the positive side, they may end up on the good end of Just Friends.

This can be Played for Laughs or Played for Drama with the girl fretting that she's a terrible romantic prospect, or such a shrew or so bad in bed she literally drove the guy into forsaking his heterosexuality. Can be played for extra laughs/tears by her having several boyfriends do this in a row, and then with Mom calling to ask whether she's "turned another guy gay".

In the least conflictive examples, the guy will realize due to outside events rather than from their (lack of) chemistry. Sometimes it'll happen after they've broken up, which may or may not be a relief to the girl. In comedies, this can be a funny way of Cleaning Up Romantic Loose Ends.

This trope has a Spear Counterpart and Gender Flip in the form of a lesbian coming out to her boyfriend after going through a similar process. A comedy will usually imply that the guy is so bad at sex or such a perverted pig that the girl becomes a lesbian out of disgust. Or both!

This trope is in a way the Inverse of Closet Key, rather than a lack of reaction from a heterosexual partner, it's a same-sex attraction leading to the epiphany. It's worth noting that some women may become Fag Hags who Settled for Gay because All the Good Men Are Gay.

Examples of Last Het Romance include:

Comics

In Persepolis, Marjane's boyfriend Enrique outs himself.

Enrique: If it didn't work with you, it wouldn't work with anyone! I never really knew who I was. You removed all my doubts. I swear, it's not you. I think you're pretty, attractive, sweet; it's me, I promise you, it's me.
Marjane (narrating): Nevertheless, somewhere inside, I was reassured. It was easier to accept that he was gay than that he had preference for Ingrid or he found me ugly.

Film

In the View Askewniverse, all of Randall's ex-girlfriends are now lesbians.
Played for sad in Edge Of Seventeen, in which a one-sided Childhood Friend Romance becomes Just Friends and then this.
The plot of Saved is set into motion by this.

Live Action TV

How I Met Your Mother: Robin lost her viriginity to a gay guy, only not really because he'd barely gotten in before he stopped and announced, "Yeah, I'm gay."
On The IT Crowd, Jen plays this to a man whose sexuality is a mystery through most of the episode. At the end he breaks down and cries that he only went out with her because she looks a bit like a man.
Seinfeld also plays with the trope. Just as George has had at least one woman become lesbian after being with him, they revert after a night with Kramer.

George: I drive them to lesbianism. He brings them back!

This is the college backstory of Will and Grace, as revealed in a flashback episode.

Will: But... Don't you see what a compliment that is? I mean, I mean, I love you, so if I can't make it work with you, then it'll never work with any woman, because you're perfect for me.
Grace: That is not a compliment. A compliment is, "You're sexy; you turn me on," not, "One look at you proves I'm a queer."

Happily Divorced averts this trope. In the case of that show, the husband has always been gay, but he doesn't reveal it to his wife until they've been married for a long time and he wants a divorce so he can live a gay life. Bonus points for being based on star Fran Drescher's real life.
Friends: Ross' first wife turns out to be a lesbian, while pregnant with his child.
On Roseanne, Arnie was obviously bummed when his ex-wife Nancy came out of the closet. His friends tried to jokingly reassure him that he was just so virile that Nancy knew no other man could compete.

Webcomics

In Blip, K was Bang's last girlfriend before he realized he was gay. They remain close friends afterwards.
Happened (only once!) to Davan in Something*Positive, though it's not explained why the girl switched teams.
Robin is this for Ethan in Shortpacked. She already suspects that he's gay, but is persuing him anyway. He's not interested. She gets pissed off when he bangs a chick, and when he asks why she shouts he's supposed to be gay. Confronted with this, something goes click in his mind. Cue gayness.
Melissa & Justin in El Goonish Shive. Melissa is also inadvertently responsible for outing him in the more traditional sense, since afterwards she told her sister, the town's biggest gossip, about it.

Western Animation

Condie Ling from Rick and Steve turned Rick gay in college (in his own words). She's since become a Fag Hag (with suicidal tendencies played for laughs) since it is inferred that he wasn't the only person to whom it happened.
On King of the Hill, Peggy—who told Hank that she was a virgin before they got married—admitted that she had once had sex with a male friend who was unsure if he was gay.

Luanne: Did you fix him?
Peggy: Oh, no, Luanne, he was not broken. Just gay. Very, very gay.