The pragmatic answer to "is it ethical" is that you won't stop riding when the nice bike is stolen or vandalised, so the real situation is whether it's better to buy a series of nice road bikes, or one nice road bike and one cheap "beater bike" that you can leave locked up in public with little risk of losing it.
A different take is that you can still be "more ethical than average" even if you own two (or more) bikes, because the average person has a car or scooter that is much bigger (and uses much more fuel and space) than even five bikes. This is where money does largely work as a measurement tool. The cost of buying, parking and driving a vehicle is roughly proportionate to the ethical costs (almost regardless of your ethical system). So if you can buy and run five bike, or ten bikes, for the same as your neighbour pays for their scooter or car, you can say that one or two bikes is almost certainly better than that scooter or car.
sustainability.stackexchange is one place to find out more about environmental ethical questions, and it would be worth asking on buddhism.se about the religious-ethical side if that's your interest. I suspect that almost any number of bicycles would win over a car from that perspective because of the numerous harms that cars do. But I know buddhists who drive cars... :)