I don't think it's so unrealistic they fell for him, when people become so close it tends to be pretty normal.
Other things are unrealistic: for example, the fact Ijichi, with her personality and contact network, never got interested in anyone else before, and the contrived set of circumstances that allow the 3 of them to get so close in the first place...
Every single one is not so weird per se, but all of them together of course are extremely unlikely in real life.
And let's not kid ourselves, one of the main reasons they feel so comfortable with him is he's the only guy who doesn't see them that way; the most unrealistic element of them all, for an etero boy in the middle of puberty.
I think we like fiction because we crave realistic stories with unrealistic circumstances, and this for me is a neat example that fits the bill.