I have written a YA romance which features a love quadrangle, and was wondering if this kind of book would have much of a market in Australia, or world-wide. To be honest, I have seen many books which have boy-girl relationships, but never any triangles or quadrangles (maybe I haven't looked far enough). Content-wise, the novel doesn't feature any sex, as I feel that under-aged sex had been done to death, and wanted to avoid this. It does, though, have some moments of eroticism, but, on the whole, the book is more pyschological in tone, with light humour interspersed.
A love quadrangle doesn't, I suspect, mean that this novel would be cast as 'genre fiction' - and, if it were, that kind of genre really only has clear markets in the US ('love quadrangle' would see it classified as 'romance novel') - so the story has to succeed or not based on the usual factors: plot, characters, quality of the writing. YA fiction in general is being published widely in Australia and elsewhere, and publishers who used to ignore it are now realising its potential (lots of grown-ups read YA too). So if the manuscript is good, regardless of the quadrangle, an agent and/or publisher will want to read it. The quadrangle wouldn't mean anyone is more likely to buy it, and probably that no one is less likely to buy it, although one can never tell. It's all just down to how you've written the story.