Porco and Reiner both, can't split them up. As for what made me fall in love with this guy, it was his sacrifice; his death was glorious, because it was selfless; then I worked my way back to his childhood and rediscovered the beauty of his character, which I see as flawless from the beginning until the end; it's not the lack of flaws that makes a perfect storyline, it's the flaws and faults that shape one into a wonder, especially in fiction, and that is how I see Porco -- as a small storyline that outshines many of the other more important ones. AoT is a huge world, with many individual stories within it, but few compare to Porco's and how complete and satisfying it is, from his childhood until his end, from a storytelling perspective.

Marcel, Falco & Gabi aside, Bertholdt possibly, as he seemed too shy & enamoured with Annie to attempt anything with anyone else; and Zeke, as I view the guy as aroace, assuming he (and the others) wasn't sexually abused as a child, which I suspect was common at that time and would have affected young Eldian candidates and Warriors, including Porco and Pieck. Reiner, I believe fucked round a little on Paradis until he realised he was gay; and Porco I HC as promiscuous, regardless of the abuse (until he got together with Reiner), as opposed to Pieck, who would have avoided sex (and perhaps tried to make herself as unattractive as possible to men). Colt, I don't really have an opinion on, as it could have gone either way. He could have got drunk one time and done something stupid he'd regret later.

I wrote once about Reiner masturbating, where I vaguely explain his thought process while on Paradis in relation to his sexuality:

https://archiveofourown.org/works/41786019/chapters/105274698

The image I have of Reiner in my mind is that of a man who's unable to acknowledge his own sexuality, so he crafts this persona of a straight man on Paradis and fixates on the most conventionally attractive girl he can find who's never going to reciprocate his feelings; even when he obsesses over her letter, he knows it's of no consequence -- she's married, but it doesn't matter, b/c everyone else perceives him as straight. Being openly gay in their time wouldn't be socially acceptable; and as child soldiers on a mission with a death sentence at the end of their term, do they have the time to explore their sexuality, or allow themselves to fall in love? No. But regardless, by the time Reiner returns to Marley, I doubt he's a virgin; I'm inclined to believe he tried to get it on with both men and women on Paradis, and had varying rates of success. Porco's presence in his life changes things. Porco is fully aware of his sexuality and what it entails; whereas for Reiner, their affair at first is a dalliance; and he doesn't overthink it, although he falls deeper into it and doesn't realise he's in love until it's too late. I find Reiner's internal conflict interesting, because canonically he struggles with self-acceptance, is impulsive and capable of self-harm; he's in denial and insecure, but cognizant of the fact that it's unacceptable for a man to be weak (and a homosexual) in their world. And if you take all of the above into consideration, it only makes sense for me to write GalliRei (or should I sai ReiPokko?) as a ship with a particular dynamic when it comes to their sex life; and from a writer's perspective, it is more appealing to me to write a tormented character who isn't necessarily going to "get better." I have no reason to believe that even in canon ... he gets better. Sniffing a letter written by a married woman he's not been shown to think of in years is not something that a mentally stable war veteran does. So Reiner grows old, for sure; and he may get involved with some younger men to whom he'll never commit; but he's not going to be in a stable relationship with a man (or woman). That's my interpretation of his character after writing Gallirei for a year, anyway. Sorry for the long answer, but it's impossible to answer this question in one or two sentences for me.