Alice and Bob haven't admitted that they love one another yet. They might be unable to just spit it out already. They might already be together, and just haven't gotten around to popping the three magic words yet. Maybe Will They or Won't They? is being milked for all its worth and it's just been a matter of sexual-tension-packed time.
Finally(!), one or both of them makes their feelings clear. It's a happy, heartwarming, romantic moment. They smile, they hug, they kiss. Good feelings all around.
Cue the sex.
Confession Causes Consummation is when the first confession of love between characters is immediately followed by sex. It's not unusual for the immediate sex to be used as the couple's moment of Relationship Upgrade or the confirmation that They Do, as they will afterwards be treated as an Official Couple. It also doesn't have to be their first time having sex, but it is their first time confessing their feelings.
This trope is very common in visual novels/dating simulators, where the romance between the main character and the pursued love interest culminates in the love confession and the sex. It's not unusual for it to be the "grand finale", with the story ending shortly after.
Sometimes a very stressful experience inflicts a Love Epiphany that leads to the confession, in which case it may also be Sex for Solace. Characters who had a hostile relationship with one another prior to the confession-and-consummation are probably looking at a Slap-Slap-Kiss sort of romance. Last-Minute Hookup might be at play if the confession-and-consummation is a means to wrap up loose ends before the story is over.
Compare Fourth Date Marriage, which goes beyond just sex. Compare this to Sex Equals Love, wherein the sex is the beginning of romantic feelings, and Insatiable Newlyweds, wherein recently a recently married couple can't keep their hands off each other. Also not to be confused with Coitus Ensues, which is when sex between characters comes out of nowhere/for no reason/grinds the plot to a halt.
- InSecurity: Hubbs and Ellie do it right after confessing to each other after spending a day in Las Vegas.
- In There, Beyond the Pass, Norn and Alice, having lingered at Cannot Spit It Out level for years, are trying to find their way in the mountains in a horrible frost. The constant danger finally triggers them into admitting their feelings, which is immediately followed by consummation. In their case, however, there is another reason for the quick development: Norn can only transfer his supernatural powers to Alice via sexual intercourse, and they both need these powers to survive.
- Green Tea Rescue; Izuku is forced to stay over at Ochako's apartment after the Sports Festival due to rain, and when they have a brief discussion and freak out over sleeping arrangements they quickly come clean that they love each other. Their kissing soon leads to Ochako on top of Izuku, but just as he reaches up to take her bra off Ochako's alarm clock goes off, waking her up from her Erotic Dream and reminding her that Izuku was too exhausted from the Festival to do much besides crash into bed back at his house. Ochako smashes her clock in retribution.
- Avatar: While trying to fit in with the Na'vi people while occupying a Na'vi body, Jake becomes closer and closer to The Chief's Daughter, Neytiri. Though at first annoyed at his helplessness, Jake's gradual growth on Pandora has her side-eyed glares turning into longing looks. Jake is finally initiated into the Na'vi tribe, after which he and Neytiri take a walk through one of their sacred forests. Neytiri suggests he pick one of the Na'vi girls as his mate, but Jake makes it clear she's the only one for him — very, very clear. Unfortunately, Jake fails to mention the impending attack on said forest beforehand.
- The Infernal Devices: Though Tessa and Will obviously have more-than-friendly feelings for one another, Will's belief that anyone who loves him will be cursed to die drives him to push her (and others) away. This includes his best friend, Jem, who loses his patience with Will's destructive tendencies, and tells Tessa as much after they have to drag him home drunk. It's during this talk that Jem confesses he holds feelings for Tessa, too. Despite her shock, the two end up in a heavy make-out session that leads to some clothes being pulled off and them gravitating to the bed...until they knock over Jem's medicine, which stops them.
- In Tempe O'Kun's Windfall Max and Kylie spend the first half of the book too anxious to make the first move on each other, then they witness a monster attack and after escaping have Glad-to-Be-Alive Sex and confess their mutual feelings.
- Game of Thrones: Robb clearly has his sights set on Talisa from the moment they meet during the aftermath of a battle, but neither of them says anything of it. After Robb is pushed into an arranged marriage with one of the Frey girls, however, a late-night visit from Talisa finally gets him to spit out that she's the only one he wants to marry. The two immediately have sex afterwards.
- Subverted in Lost, since the love confession is heavily implied but not word-for-word: The infamous Love Triangle between Kate, Sawyer, and Jack is filled with a lot of pining, longing looks, sexual tension, etc. Sawyer in Kate in particular have bucketloads of Belligerent Sexual Tension up until they are kidnapped by the Others and locked in cages. Kate is adamant about getting her, Sawyer, and Jack back to the safety of their camp, not knowing that they are no longer on the same island. When pressed on the matter, Sawyer, rarely softspoken, confesses that he never told her because he wanted her to think they had a chance of living.
- Subverted in Riverdale, with the sex implied this time: main characters Betty and Jughead become an Official Couple after loads of Ship Tease in the sixth episode of the first season. The two remain together despite the stressful situations they find themselves in, from their family issues to investigating the murder of Jason Blossom. It takes until the season one finale for them to finally say the words "I love you" to one another—it's heavily implied that had the Serpents not showed up on the doorstep there and then, they would have had sex.
- Life Is Strange 2: The third episode gives the player the option of having Sean pursue Finn or Cassidy (or neither). In the latter's case, going for a late-night skinny dip in the lake leads to the opportunity to tell Cassidy how Sean feels. Joining her in her tent after is also optional.
- Stardew Valley: The ten heart events of most of the marriage candidates involves a Fade to Black just as they confess they really love you, though it varies on the candidate as to how much they've said already (they are explicitly dating you after all) and up to the player whether anything actually happens when the scene ends.
- DRAM Atical Murder has this in both Koujaku and Noiz's good endings. The confessions of love from either of them are quickly followed by them and Aoba having sex.
- Katawa Shoujo: In Lily's route, Hisao and Lily have sex minutes after Lily confesses to him.
- Seduce Me: In any of the boys' routes (James, Erik, Sam, Damien, Matthew), it'll come to a head when Diana makes an offer for them to return to the Demon World with her and leave Mika (the default name for the player) behind. To get the good ending, Mika interrupts to say she loves the demon in question. Said love will be returned if enough of the right options are picked. In this case, the sex that follows is entirely optional and won't affect the happy ending.
- A variation occurs in chapter 7 of Long Exposure in that they're already a couple when this happens. After Mitch tells a doubting Jonas the reasons why he loves him, they have sex.