AITA I don't want to pay for intercontinental flight seats

So I purchased 2 tickets for me and my significant other. They costs $2,000 for economy - not too bad for direct flights from the US to London. I did not choose seats and pay those fess as they cost $700 to $950 ADDITIONAL.

Significant other asks me to add the seats on for $700 more. I say no thank you. I offer a compromise: You can pay for that portion of the trip and pay for the seats. She did not like my attitude about it.

AIAH? The trip is my idea. In about 80% of the cases, we end up sitting next to each other anyhow picking fee-free seats day of travel. A plane is a bus with wings for me. The cheaper we travel, the more things we can afford to do.