How does an aromantic even get to the point of being pressured into a marriage (at least in a society without arranged marriage)? Why are they dating in the first place? Am I misunderstanding how that works?
Mainly social expectations and lack of awareness of aromanticism. I know in the US that's common in the deep south (where I'm from), but I'm sure you'll find it anywhere that's socially conservative.
Could be family pressure. Could be internalized hetreonormativity making them feel like they 'should' do this. Could be they haven't really realized, come to terms with, or accepted their own identity.
I mean, think of a 'stereotypical' aromantic guy. He's interested in women, and sleeps around a lot, but despite not getting feelings, might 'settle down' with one partner because its 'normal, respectable', even if it's not something that makes him happy. Probably won't make the wife happy either, but that's it's own issue, why she might marry a guy that 'doesn't do romance'.
The prefix Allo just means other, so when you have a pair of things the other one will normally become Allo-thing. Because we don't make words the culturally accepted default position until there is something to contrast it with, most instances of Allo will describe the culturally accepted default.