You do realize that if people followed canon sexualities 90% of ships wouldnt exist right?
There's not nearly that many canon gay characters and gay fanships for that ratio to be close to accurate.
Just looking at Kantai Collection, you have only a tiny handful of not-quite-confirmed gay shipgirls (just in case shippers want them to be straight), and most are canon straight... and the vast majority of fanworks have male admiral/shipgirl relationships. Just look at the disparity between "admiral_(kantai_collection)" and "female_admiral_(kantai_collection)" for that matter. (Because the male version doesn't need a gender qualifier - it's the default.)
Even Touhou, which is practically made for gay shipping, doesn't force Rinnouske or (presumed male) "reader" characters below 10% of the fanwork relationships.
It's absolutely true there's people who will force gay shipping into anything they watch, but there's far more people who are going to force straight shipping into anything they watch. It's just that there hasn't as yet been that much canon gay to require any "forcing".