You have the right to block or ignore or stop talking to anyone who messages you via online dating.
You don’t have an obligation to justify yourself. You don’t have an obligation to worry about their feelings.
Others have the right to ignore you, and you can ignore anyone you like. Even if they are polite and respectful. It is unreasonable to expect people to respond to every query and solicitation.
If you get a message, and you are not interested in the person - even if it is for the shallowest of reasons (you don’t find them attractive, you didn’t like the content of their first message, they like cats more than dogs) - you can simply ignore the message. You can delete it. You can even hide or block the person if you like.
You don’t owe anyone your time.