but in this case
that's the first time they've seen each other since YEARS
and she was FULL clothed lol
For him to have that erection like that he must had a lot of fantasies with her, to the point that's what he thinks first when he sees her
I don't think that's the normal reaction to most people but who cares
Well when you put it that way, yes I can agree with your point of it being weird. I think that in the last chapter he actually said that he has had fantasies about her so with that we can assume it didn’t take much for him to have an instant erection. Tho with this context I do agree with you, it is weird lol

instant erection? wtf lol