"Adjustment period" takes years and isn't completely over by the late teens. And, as I already said, for a healthy male it's impossible to fake ED. Ever.
I also have a sister, we grew up together. And I'm telling you,
it really doesn't work like this. You may get used to one girl eventually and some situations, like sauna for example, but dropped into a different situation with someone else and this is not what you're used to any more. The main point is, erection doesn't work at will, especially for mind numbingly testosterone-loaded teenagers.
But I'm not going to keep explaining how a male erection works or what it's like go through male puberty to someone who clearly hasn't experienced one or simply talks bullshit, but who also completely ignores
the major fact that Nika is
the main character in this story and
we can read his thoughts all the time.
So you don't like what the story says and you create a narrative in your head that completely ignores the facts already established by the story and creates a
main character instead who is not just an incredibly well sexually trained
virgin going through his late puberty, but also someone who lies in his thoughts
to himself about his PTSD, complete sexual apathy and
fakes panic attacks so well that he believes in them himself...
In-cre-di-ble. Nika really needs some serious therapy, doesn't he?!