You know that the Constitution isn't the only law you have to obey. There's nothing in the Constitution about speed limits, no parking and handicapped parking zones, social security,
If the Constitution hasn't granted Congress the power to pass those respective laws, then they're unconstitutional and will be unenforceable in court. (In practice, the President appoints judges, so over time they usually get their way regardless of what it says.)
The point is, I never signed any contract as a business owner compelling me to serve anyone.
If I sit down at a restaurant and the waiter delivers my order, that's an implicit contract.
If I walk into a supermarket and see a cake "on sale until Tuesday, $19.99 while supplies last", that's an contract.
If I walk into a bakery and ask for a quote for catering, they're under no obligation to serve me (until I accept the quote, that's an explicit contract.) They can turn me down for whatever reason strikes their fancy.
But if you really think businesses should serve "everyone", how do you feel about forcing a Jewish bakery to cater for neo-nazis?