Marriage Toxin (or Marriagetoxin?) is about assassin Hikaru Gero, who—along with his sister, Akari—is a potential successor to a prestigious family of "Poison Masters." At the beginning of the story, both Hikaru and Akari are being pressured into marriage for the sake of continuing to propogate the clan. Akari volunteers herself to do it, but Hikaru objects on the grounds that Akari would be sacrificing what actually makes her happy: Akari is in a relationship with a woman. Let us note that a lesbian relationship being legitimized and valued by the protagonist earned some favor from us. Is that shallow? We thought it was sweet.
Anyway, now that Hikaru has shut down his sister's self-sacrifice, he is the sole candidate for succeeding the Poison Master family. The problem, though, is that Hikaru knows his way around a wet job and not much else. Suffice it to say that romance is not his forte. Enter "marriage swindler" Mei Kinosaki. When Hikaru rescues Mei from a gang of criminals, they seem to hit it off, and she agrees to work with him as his "marriage advisor," assisting him in finding damsels to rescue and woo. Oh, but Hikaru learns something else about Mei at the end of volume 1: she has a penis.
Allegedly, Mei presents as whatever gender she needs to for the mark she is trying to con at the time, but we effectively never see Mei present as a man, so the math doesn't quite work out there. Pair that with the fact that the translators degender "him" by using they/them pronouns, and... well, it smells like a transgender woman to us. There is nothing innately wrong with Mei being trans, obviously; she's a talented, charismatic, and overall likable character who comes to support Hikaru as a friend. We don't feel that Mei is intended to be a negative transgender caricature, but she is evoking transmisogynistic stereotypes. Consider, for example, that her entire job is to deceive others by "acting" as a woman. To say the least, we have mixed feelings.
The general structure of the first few arcs goes like this: Mei finds Hikaru a viable damsel for courtship, he rescues her from a series of gimmicky goons, she is endeared to him, and then they exchange contact information. Rinse and repeat. It's kind of like he's assembling his own harem anime cast. The female characters are charming, and above being archetypes, but only slightly.
At times, the manga does feel like it's trying to tell a story about meaningful relationships that challenge traditional conservative ideals. You can see this in Akari feeling like she needs to sacrifice her queer relationship for the good of her family. In one scene, a young Hikaru is visibly uncomfortable while being taught about the "typical" heterosexual family structure. Another character, Piichi Nakagawa, nearly fights his girlfriend's mother to the death challenging her patriarchal ideals (this happens in a oneshot; it's actually pretty good).
In the hypothetical best version of this story, Hikaru nearly loses Mei, and then realizes that she's what he wanted all along. He fights for her once more, (somehow) they find a way to overcome the expectations of the Gero family, and Mei gets a happily ever after as the heroine of this manga. Somehow, we don't think that's what will happen. We can't shake the feeling that this all has something to do with Japan's declining birth rate. But we can dream.
If Joumyaku fully commits to a narrative that radically challenges conservative relationship ideals, then more power to him. I'll freely tell you one reason we're not sticking around to find out: it's a fucking battle shonen. As we previously alluded to, each antagonist has their own gimmick (justified in-world by assassin families having titles similar to Poison Master, i.e. Beast Master, Water Master) that they use to try to get one over on Hikaru. Hikaru, in turn, finds a way to persevere and power up by poisoning himself. Then the next guy is stronger, so—so Hikaru has to, like, poison himself more betterer. Whatever. It spends a lot of time on this type of ever-escalating violence, and we didn't care much for it. if superpowered shonen is your thing, and any of this sounds interesting to you, maybe you could give Marriage Toxin a look. You could do a lot worse.
You could probably do a lot better, too. We dropped it because we were bored.