Platforms like reddit are in no way similar to personal property like a house that you live in.
A better analogy would be, imagine you rent your house to someone else. You make a rule that tenants may display political messages in their windows, but only for one political party.
That would be illegal. You can prohibit all signs if you want, but specifically choosing what signs someone gets to display violates their first amendment rights and could trigger a fair housing lawsuit. It doesn't matter that you aren't the government and that you own the property.
The renting analogy fits even less though. Renters have protections against evictions that don't exist for websites. If I break the rules of my lease it would take a month or 2 minimum to get kicked out. If I break Reddit's rules I can get banned immediately.
Sure, when “talking” is done via sound waves or someone else’s medium it’s different
But if you have free-speech parties at your place and people spray paint messages on your house then you are now “hosting”
You decide which words you leave or paint over