It's not the same thing.
Okay, look, as a business of public accommodation (a business open to the public), they are required to not discriminate against customers. That is to say, they are not allowed to refuse service to a customer on the sole reason of that customer falling in a protected class.
Federally protected classes include, but are not limited to, race, age, sex, national origin or ancestry, religion or creed, and physical or mental disability. (I may have missed a couple in there.)
States are allowed to add to the federal list, but they are not allowed to remove anything from the federal list. In the state that the "gay wedding cake" occurred in, sexual orientation is also a protected class. So, the bakery in question was not allowed to deny them service solely on that basis.
Now, if the bakery had been booked solid, and would have been unable to produce the wedding cake by the time of the wedding, and they'd denied service based on that? Not discrimination.
If the bakery didn't do wedding cakes at all, as in it wasn't a service they offered, and had denied service based on that? Not discrimination.
Now, Reddit probably doesn't count as a business of public accommodation. While anyone can read most Reddit posts, you have to login to comment. It requires membership to access certain services. They are free to set the Terms and Conditions under which they will allow those services. And they are free to say what constitutes a violation of those Terms and Services. (Although, let's face it, while they should be consistent in that application, it's practically impossible to prove that.)