Ships

Yuuji hates Mahito, Mahito loves Yuuji. Not a good kind of love where he wants to go on a nice dinner date but more of a "I'm going to break your psyche and hurt you so bad that you will never be the same again. I'll never leave you, even if one day you do succeed in killing me" kind of love. Their relationship is fully based on violence. Everytime they meet it's detrimental to Yuuji. Yuuji is able to deal a whole lot of physical damage to Mahito though so he never gets out unscathed either. It's somewhat theraputic to Yuuji, beating the shit out of Mahito feels good to him but he would never admit it outloud. Or even silently to himself. He's convinced he'd be a bad person if he did. He hates seeing Mahito, he hates the person he becomes when they fight and he hates the person he becomes when they fuck. He hates Mahito and he hates himself. Mahito on the other hand enjoys toying with Yuuji, sure he can get pretty badly damaged but he can always regenerate after! Mahito's life becomes this little game after realizing that Yuuji is for him. Mahito is for Yuuji. He lives to destroy Yuuji. Yuuji lives to be destroyed.
Sex for them is fairly animalistic. It's feral and carnal. Bloody and disgusting. Yuuji usually doesn't/can't speak during it and most of the time is sobbing or focused on something. The most Mahito will get out of him during sex will be unintelligible words or very broken strings of apologizing and asking for forgiveness. (He isn't asking Mahito for these things, they're meant for himself/no one. He just needs to say them) Mahito is moderately vocal during sex, usually to insult and belittle yuuji, telling him that he's a horrible person and a horrible sorcerer. That he's letting everyone down and that he deserves the worst to be brought upon him. Intercourse doesn't always follow through this way, Sometimes Yuuji will take the lead, though it's even more violent and damaging to Mahito than it is when it's the other way around. Sukuna moniters their interactions but not to help. What Mahito does is a little entertaining to him, he mostly enjoys what it does to Yuuji though. He doesn't really gaf about Mahito. In fact it irritates him that Mahito thinks that he'll be the one to break Yuuji.

Sukuna has no sexual nor romantic interest. He doesn't care about having sex with Yuuji, though one time while Yuuji was dead (see here) he had Yuuji get off on his shoe just to mess with him. Yuuji's teenage body amuses him, he hasn't felt that way in centuries so it's slightly nostalgic.

Under significant revisements

Izuku's relationship with Aizawa is probably the most healthy relationship he has in his life. Most of their interactions are when he would come to Aizawa and break down in front of him, the stress of it all collapsing onto him. After the first few times he's been able to tell when it starts getting worse for him and he can go to Aizawa prematurely so it doesn't get that bad. Aizawa being the only person who he feels the most comfortable with being vulnerable around and knows about one for all. Aizawa lets him cry on him, telling him that it'll all be alright, sometimes comforting Izuku for hours. The visits become more frequent after the war, happening at least about 5 times a week, juxtaposed to before where it was once a month. Izuku becomes a little dependent on Aizawa, of course he's careful not to expose his "relationship" with Aizawa but it's hard for him to be seperated from him for a few hours. He feels relaxed when Aizawa is in the room. Outside of those times Izuku can be dissociated. Aizawa helps pull him back to reality. He needs Aizawa now, this no longer something he'd occasionally need. (None of this is romantic, they never confirm what they have and Aizawa has no romantic interest in Izuku. Izuku's feelings are more complicated than simply calling them romantic feelings..)

Izuku's mom probably kissed him while she was stressing about pretty much being single mom. She still feels guilty about it to this day even though Izuku doesn't remember it at all | All Might kissed him to mix saliva or whatever Idk man

This only happens after Twilight is made into an alicorn/princess. I believe that Twilight really didn't want to be either of those things and Princess Celestia was wrong for not asking her beforehand, taking it upon herself to change Twilight's body + social class FOREVER. In my mind Princess Luna and Twilight start talking a lot and begin bonding over their shared experiences and emotions on Celestia fucking them up to an insurmountable amount. (Keep in mind that Celestia had been grooming Twilight for the position of princess since fillyhood as well) Eventually they become so close that they start dating, though their relationship is unhealthy and heavily codependent, using each other to try and cope with it all. Of course they are in love too but it isn't good for either of them. Princess Luna has doubts about their relationship, focusing on how their age gap is so wide rather than how attached they are to each other. Twilight is head over heels for Princess Luna and sees nothing wrong with their relationship for the most part. Though sometimes she does start panicking and paranoia sets in as how everything about them is wrong, she pushes those times away and tries not to think about it. Princess Luna attempts to break up with Twilight, though her attempt is futile. She still loves twilight. She still needs twilight. Twilight still needs her. They never break up again, even for a few moments. Princess Celestia doesn't ever find out about their relationship, I don't think she cares that much about whatever Twilight does in her free time if it doesn't impact her ability to perform as a princess.