The aircraft that I was the only passenger on, had to fly to my destination anyway, whether or not I was on board, because the flight it was due to operate next was almost fully booked. My presence on the aircraft was not the only reason for the flight.

In answer to the underlying issue you raised: there isn't a clear answer about whether frequent air travel is worse for the planet than owning a dog or cat.

Frequent air travel does have environmental impact. However, every other activity that humans do, also has an environmental impact. Although I feel it's important that climate change activists are making people aware of how human activity is harming the planet, I disagree with their focus on air travel. I feel they should discourage other activities, like shopping at "fast fashion" retailers, and pet ownership, instead of only discouraging air travel.