Glass no Megami

It's love at first sight for college students Sakura and Kurimiya, but Genki Girl Aya - Kurimiya's classmate since childhood and now attending the same school as Sakura and Kurimiya - somehow sweeps Sakura off his feet before he quite knows what's happening. She declares that they are now girlfriend and boyfriend, at first mostly for lots of sexual horseplay, but soon ends up falling deeply in love with him. Sakura can't believe that someone so sweet and pretty could love him so much, yet can't get over his feelings for Kurimiya. Kurimiya is happy for her friend Aya, as she sees how happy Sakura makes her, while trying to deny her feelings for Sakura for her friend's sake. In the end, Sakura has to decide what and who he wants the most and whether or not hurting and losing Aya is worth the risk that Kurimiya may not accept his feelings.

It seems like it would be a fairly standard Love Triangle plot with plenty of steamy sex, (and there is plenty of that) but the story may surprise you. It's well-written and illustrated, with plenty of drama and tension, and the characters are engaging and get enough development that the ultimate breaking of the triangle winds up being a heart-wrenching Tear Jerker.

Tropes found in this manga

 * Attractiveness Isolation: This is treated seriously with Kurimiya. She says that this, combined with the fact that she has spent a good part of her life moving around abroad, makes her feel lonely and isolated and always an "outsider" with no chance of a real relationship.
 * Bittersweet Ending: One pair gets a happy ending, the loser goes through pain and suffering that can unintentionally offset the happy side. However, at the end, she seems to be recovering and pursuing her dreams.
 * Cannot Spit It Out: Kurimiya tries to deny her feelings for Sakura for her friend Aya's sake. Because she wants to hide her feelings from him, Sakura doesn't realize that she feels the same way about him as he does for her, which prevents him from coming out and telling her he can't think about anyone but her.
 * Everyone Can See It: Pretty much everyone can tell immediately that Sakura and Kurimiya are mutually attracted to each other. This includes Aya, who either won't or can't bring herself to believe it.
 * First Girl Wins:
 * Genki Girl: Aya, Turned Up to Eleven.
 * I Want My Beloved to Be Happy: When, her heartbreak is visceral and painful to see such that it deconstructs the trope.
 * Stripper With A Heart Of Gold: Shiratori. She is willing to give Sakura relationship advice while blackmailing him for her own use.
 * Love Triangle: Kurimiya and Sakura fall in love at first sight. Aya grabs Sakura seemingly on a whim but winds up falling deeply in love. Sakura tries to love Aya back but all he can think of is Kurimiya. Kurimiya and Aya are long-time school friends and Kurimiya, despite her own feelings, wants Aya to be happy.
 * The Matchmaker: Sakura's cousin, Jun, sees Sakura and Kurimiya's mutual feelings right away and does her best to pry Aya away from him so he can pursue Kurimiya.
 * Porn with Plot: There is plenty of raunchy sex, but the plot is well-written and the characters get plenty of development on their own, becoming people you care about.
 * Renaissance Girl: Kurimiya is intelligent, an excellent cook, speaks English, spent a few years in Europe while growing up, shows interest and knowledge in art, movies and literature, loves listening to Sakura talk about his chemical engineering classwork just for the chance to "try to learn something new", gorgeous and very sweet-tempered. Sakura has a hard time convincing himself that someone like her would actually be in love with a nerdy ChemEng student like him.
 * Sempai: Aya calls Sakura "Sempai" more often than by his given name. (Even when they are doing This and That...) Sakura and Kurimiya are both Aya's seniors in school and Kurimiya explicitly says Aya is her Kouhai.
 * Seinen: The far end of it in terms of strong sexual content, so much that many thinks that this series is actually Hentai; the truth is that it was serialized on Vitaman, a genuine Seinen magazine.
 * Shallow Love Interest: Deconstructed with Aya. Aya is a truly sweet, kind and caring girl who loves Sakura -- a lot -- but he can't stay interested in her as a person and she can't really keep up with his academic interests. On the other hand, Kurimiya can actually hold a conversation with him on things he feels passionate about, finds what he has to say interesting, and he in turn finds her to be interesting and enjoys just being around her, completely aside from the physical attraction.
 * Shipper on Deck: Sakura's cousin really wants him to hook up with Kurimiya when she visits.