Since its a Chumra only observed by some, Others think its a despicable “custom”
Yes, but you are being unclear.
Nobody (should) think that it is a despicable custom in itself. What you mean is that some people don’t believe it is being done for religious reasons, and instead think it is being done to degrade women.
For obvious reasons, that is an absurd accusation.
Since you said its not Halacha, there is no need for those who dont follow this chumra need to belive it in
Certainly. I personally don’t know that I would only take a separate seating bus, even if it was available. I would probably just take the first one that came.
Why should those who think its disgusting observe it?
They shouldn’t. They should, however, have enough respect for their fellow people to allow them to do their customs within their own domains.