Does a buy have to have a realtor to buy from a realtor?

[John Lindemann]

I am selling an investment house in Oklahoma and I have signed with a realtor to list and sell my house.  Last weekend a buyer found our house from a listing my wife had posted on a Facebook page.  The buyer messaged me and then we spoke on the phone and then I showed them the house since I was going to be there anyway.  They are not using a realtor and want to negotiate with me on the price to reflect that I won't be paying a buying side realtor the 3%.  I anticipate receiving an offer this evening from them.

I spoke to my realtor and he said that they had to be represented by a realtor since it is listed with a realtor and is on the MLS. Is this true? He offered to do their side at a reduced commission of 2%.

I genuinely like my realtor and don't want to mess up my relationship with him but don't understand why the buyer has to have a realtor or why I'd pay an extra 2%.

Any insight would be greatly appreciated.  I'm looking for reasons other than "if you like your realtor you should pay him."  This is real money and if I'm going to pay somebody I need to know what I'm paying them for.

Thanks so much for taking the time to read my post and provide your feedback!