So far? yeah it is. But honestly, way too soon to tell to be honest. There are things to criticize, of course, and so far there is barely any plot moving forward, but overall is quite absorbing. You want to know how it continue, you want to solve the things in the novel, you want to immerse yourself in it.
A couple of things about your comment.
1) missing girlfriend gives you erectile disfunction. People react to trauma differently. It's quite clear he is just no interested in sex at the moment given his depression/state of mind. This is soo standard that I bet you may know someone just like that. You are confusing the erectile disfunction of a person that is all ok and cannot get a boner with someone that, and explicitly stated, can get a boner if he thinks of the girl he loves, problem is the bitch is dead. Depression can really fuck up a person.
2) He is not saving himself for her. First, it is more than hinted the "summer's gone" is not something for "summer is gone but she will be here again in chapter 24". It's "summer's gone, deal with it". Second, Nika explained it himself he would want to feel stuff for other people, he just can't.
3) About letting Jeff get into someone's panties. I am confident that girls you don't choose will end up with your friends. It's the nature of a more realistic novel not for all girls to be there just to talk with the MC. Having said that, so far the only one I approve of is Mom's boyfriend. Unfortunately, for all I wanted to hate the motherfucker fucking mommy, he seems like a great guy. For now. If he does anything not as good as what price charming would do, we need to kill the bastard. All others are in discussion.
If you ask me, there are scenarios in this novel where MC having sex makes sense. No now, but later. After all, as you said he is in the pinnacle of his life and hopefully we will get to a moment in the story where his mind set is ok for it.
But my overall point is that you could simply do what they do in movies: show you the start and show you the after event. No need to show the sex scene for this novel to work. For example, take Pale Carnations. You cannot remove sex scenes from Pale Carnations because they are at the core of the story. But you can remove them altogether from SG and no effect on the novel itself if you are just hinting them.
DISCLAIMER: I know Nojiko is not his biological mom. But mom for me is the person that raises you, not the blood related person that creates you. So Nojiko is MC's mom in my head. Same idea with Nami.