Honestly this just makes me think of T4T. Like, allosexuality/alloromanticism is unfortunately seen as the default/norm in society, and I'd absolutely understand that when someone deviates from a given norm in a way that a lot of people don't understand or see as valid, that that person might prefer to stick with people who are known to be "safe" — that is, people who share that experience in particular. This holds true for a lot of traits, not just being demi.

So if someone turned me down because I'm allo, and that person only dates other demis, then it really wouldn't really register to me as "allophobic". I might be disappointed or find it a shame, but I'd understand it and find it just.

Allophobia is a thing that happens sometimes, but I really don't think this could be cited as an example of it.