The Constitution guarantees equal protection under the law.
But the problem with your claim is that this involves “Government” involvement, not private people. Now I can certainly see a difference between large companies and individuals so I will give you that the large company is more directly part of a system needing Government controls but not the individual.
The denial of a “cake maker” is not based on selling a cake, but the need to be directly involved, intimately, in the event and people who are celebrating a lifestyle that goes against deeply held moral beliefs. The “cake maker” is not refusing to sell a over the counter cake, the “cake maker” is not refusing normal commerce in any way, shape, or form. Their only complaint is being forced to directly take part in a celebration, being included in an event they find morally wrong.
Now this is in contrast to my view that those who “WORK FOR THE GOVERNMENT” refusing to do their jobs as representatives for the Government. For example, if a Government worker refuses to process wedding licences out of a claim it offends their religious beliefs, those people should be fired, on the spot. Why? Because they made the “choice” to step into the role of Government representative and once they step into that role, they are part of the system and must do the Government’s business, if this is too painful to do then quit, the Government can’t cease to do business.