A group of people walk into a cake shop. One is Jewish. One is Homosexual. One is Colored. One is You.
The United States Declaration of Independence: "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."
The First Amendment to the United States Constitution: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
Per the Declaration and the Constitution, these people are Equal before the Law.
If making a cake is an activity performed as part of a publicly-recognized business conducted under the protection of the laws of the land, then the owner must accept all the laws of the land, not just the ones that can be used to serve their selfish interests and fears.
Some say it should be "left up to the business owner". I say that's passive-aggressive douche-baggery by hypocrites who can't admit to their own bigotry.
And if you don't like it, the existence of the First Amendment means that you can speak, assemble and petition for abolishing the First Amendment. I hope you can appreciate the irony.