Feel like this is sort of stating the obvious - in any context where there is a middle seat, they're often the last to go.

Single passengers or not, middle seats are usually the least desirable option, which is why they tend to be booked last.

Certainly book yourself in an aisle and your SO in a window. If someone ends up booking the seat between you, I guarantee you they'll be happy to switch with either of you if you really want to sit next to each other.