As an unmarried, childless playboy of forty-three, Henry never thought he’d be the one responsible for keeping a teenage boy from suicide.
It’s one of Henry’s literature students, a troubled boy named Marc. Over time Henry coaxes Marc into sharing his fears: He is haunted by a terrible story from an obscure book that his parents read to him, over and over, when he was a child. Henry is a professor of literature; he has book on a shelf in his office, and Marc is convinced Henry is the only person who can understand what he suffers.
But Henry is an irresponsible person and his sexual gallivanting just came to bite him in the ass. [how?] As Marc appears in Henry’s office, then at his front door, Henry spends his days trying to make up with the one person he ever loved [who? I like that you've pared it down to focus more on the conflict between Henry/Marc and stripped out some of the ex-boyfriend stuff, but I think we do need a bit more information in order for this to hit properly] and his nights screwing a woman he hates and who hates him back.
Even so, Marc refuses help from anyone else. So Henry tries to set aside the chaos of his own situation. He gives Marc the spare room in his apartment, he becomes the parent Marc needs but never had. But for the first time in his life Henry is heartbroken; his carefree, hedonistic lifestyle has become a thing of hate. The stress is eroding his body and his mental health. If he can’t get his own life in order, and do it soon, then he’ll fail Marc completely.
LOVE [not real title] is a 100,000-word literary novel. Author bio, comps, etc.
From this, it sounds as if Marc is the more interesting character. Being obsessed with/haunted by a book is more interesting than relationship problems IMO. Is it a dual POV? If so, I wonder if you could almost focus on Marc in the query. But I know that might not work for you. I agree with the previous commenter wondering what is the main plot. I think you either have to focus completely on Henry's relationship conflicts and leave out Marc entirely, or focus completely on Marc and not mention Henry's "sexual gallivanting" (nice phrase btw) because in this version it kind of comes out of nowhere with nothing to back it up.
Hope that helps! I updated my query too if you want to take another look.