
I’m asexual and it’s a lack of sexual attraction to ppl. It varies but I personally still have an interest in sex. I just don’t desire it. I assume you know this and I could see why you might say Sou is asexual but I think Sou is just very nervous. Hence the trembling. It can be scary doing something new especially this even with someone you love. He was so nervous to even tell him his feelings to the point that he said he would throw up and after being kissed unexpectedly he actually did. So I think he’s just like he said a late bloomer. He needs to take it slow.

You're right, and I also think the author is totally portraying Sou as a late bloomer, nerve-wracked virgin!! I just postulated my theory because I lived a very similar situation, and I thought it made perfect sense with the information we know about both characters. Personally, I don't believe I'll ever see an ace main character in BL, sex-positive or not, but it's nice to imagine....
Ok, not to be tmi but already tming... Chapter 4's bed scene, when they stop what they're doing because Sou doesn't feel like it but at the same time he feels like he SHOULD be doing it has happened to me (and my partner) exactly like that! More than once!! And while it's wonderful I have such a respecful partner that's not my point
My point is that I think Sou is asexual