Even though the ending was kinda good, why did tsubasa forgive the seme so easily for his ...

I read this over 10 years ago. With a very young lens, I just accepted this story (because everybody had flaws right, nobody's perfect) and enjoyed it. Now, I got to re-read the story with much acquired maturity and I gotta say, this story is actually very disturbing. Pedo-ish content aside, Tsubasa accepted cheating, lying, manipulating, conniving ass of a partner (Eiji), who can screw him over anytime Eiji wanted. I was so sad about that. Because Tsubasa was so innocent and naive, he felt that the burden of blame was on him (he was inadequate, a 16 year old could not commit to having sex with a 35 year old). Sigh

This is definitely that old trope of "I'm going to do something good I'm leaving my lover completely in the dark until the results are fulfilled" mixed in with the other trope of "I like making my lover jealous."
Yeah, this aged poorly. I think we're thankfully out of that age of confusing cool men with toxic traits that the "redemption" of "oh, I don't like seeing my lover feel negative emotions they did something for me, and now I'm the ass" is too short. We grew expectations.