It had a lengthy blowjob sequence and was part of a tankoubon
There's this twin-tailed flat/small-chested blonde (who looks like Suzu from Seitokai Yakuindomo) , she is in love with her female classmate, then she discovers that her crush already has a boyfriend with whom she has a BDSM relationship.
She accidentally comes upon her crush performing fellatio on her BF and the blonde is then seduced into having sex with her crush and her BF at the same time.
The guy puts a dog-collar on her and forces her to perform fellatio on him too, and at first, she agrees to do it only because the guy's dick is covered with her crush's saliva but eventually she finds herself secretly liking it, despite being deepthroated by the guy, and she ends up giving a double-blowjob with her crush to the guy.
From there, it continues to full-fledged sex and she actually gets mindbroken but she still continues to act tsun-dere anyway toward the guy.
There's a gag-panel where she express shock at being made to join and it's one of those distinctly-manga-style panels where her mouth is open in shock and all her teeth look like they are pointed like a shark.