
I finish the sentences like this:
"All I've done is spoiling/dirtying you" .. like making Doumeki's personality less pure/ruining his good qualities..
"If I hadn't met you I would be.. pure?" I'm not sure about this one.. I think he could say that he is more impure now, as he seems to think of his desire for monopolising Yashiro as something bad, since he compares these feelings to his father's behaviour..
on the other hand, he disagrees with Yashiro when he says that Doumeki became impotent, because he is pure. Therefore I think Doumeki may just say that if he hadn't met Yashiro he would have stayed impure, because meeting Yashiro has made him move on and giving him something to strive for.. overall made him a better person, but he doesn't seem to think of himself very highly, which is why I'm confused..
but I guess the upcoming chapters will clear this out :)

I think he's trying to say that he would have stayed dead on the inside if he hadn't met Yashiro. He felt guilty all this time and was unable to move on from his past. He was hollow, trying hard not feel anything anymore. Yashiro gave his life meaning again. Yashiro is the exact opposite of Doumeki, accepting his past without blaming anyone. Doumeki admired that in him, and it gave him a new perspective. Yashiro also made him able to feel again, so basically meeting him brought Doumeki back to life.

I think I would agree with the "dirtying" instead of "spoiling". Yashiro says that Doumeki is impotent because he was pure. But because Doumeki is no longer impotent, Yashiro is telling Doumeki that he made Doumeki impure/dirty. He (Yashiro) thinks himself as a dirty person.
And then for Doumeki, I think that he himself never thought of being a pure person. In the line "If I hadn't met you I would be..." this line is more like him telling yashiro that yashiro is not a dirty person. Telling Yashiro in a roundabout way how important Yashiro's existence is to him.

Spoiling can mean to make something worse than it was, which was what I was trying to say ;)
have you seen that they changed the line "if I hadn't met you, I would be.." to "I had to meet you to.."? Actually the whole translation of the chapter has been changed a lot..
but I guess the meaning is the same in these two sentences, as it's still about how important Yashiro is to Doumeki..

Gosh you nailed it! Total opposites, indeed. Yashiro moved on but he was self-loathing; Doumeki caught in the past, his impotence was the proof of it, and kept blaming not only himself but his Dad, as well. Though, he hasn't outright mentioned blaming his father but preventing himself to become like him is kind of saying so. Yashiro thought he was tainting the "pure" Doumeki just because D couldn't have sex; D is being freed by Y from the dark past caging him in. Up next is to free Y from self-loathing. One bird down, one bird to go.
I always tried to answer the same question Y and everybody else had: What exactly did D see in Y? Now it's answered but D fell in love even before knowing the whole story, does that mean the attraction started with the classic love-at-first-sight?

I wouldn't say that Doumeki fell in love with Yashiro at first sight, but just that he liked his looks and that he could sense that Yashiro was somone special. However, if I remember correctly, Doumeki realises at one point how hard he has already fallen for Yashiro, so I would say he liked Yashiro to begin with, but fell in love with him after getting to know him :)

After reading long posts at domeyashiro (way after I wrote my comment), D had respect for Y. I guess because Yashiro never looked like the typical yakuza. Even before he knew that Y was a yakuza, Y just had a different aura, someone who always looked forward and seems carefree. Different to how D lived, before and after the incident with his Dad. So yeah, my bad on that question.
how would you complete what yashiro says here? "all I've done is...." http://www.mangago.zone/read-manga/saezuru_tori_wa_habatakanai/mf/v05/c024/29/ and here, doumeki's sentence? "if i hadn't met you i would be..." http://www.mangago.zone/read-manga/saezuru_tori_wa_habatakanai/mf/v05/c024/30/