This issue isn't so cut and dry.
Here is a case where the baker won at the Supreme Court over a wedding cake but found himself in court again where, in my opinion, he was just being targeted.
Religious convictions do matter.
While you are correct discrimination is wrong, not wanting to provide a certain service to a gay couple isn't necessarily discrimination.
Personally, this is another area I don't believe "gay marriage" is necessarily part of "gay rights". Gay people deserve all rights and privileges provided by "marriage", but it's not a given "marriage" is the only way this must be accomplished. Protections of relationships and recognition that 2 people are legally a couple is a right as far as I'm concerned. But "marriage" isn't necessarily the way it must be done. "Marriage" is not a gay right. Legal protection is.
Interestingly, Gay Marriage became federally recognized not so much because it was determined gays are entitled to marriage. What the Supreme Court said was states must recognize same sex marriages that were done legally in states that do support it.
From what I've read, Gay Marriage exists nationwide de facto. It came in through the back door, so to speak.
Now the question is will churches that do not recognize same sex marriage and certainly not permit it be done in their sanctuary be forced to accept it and make it part of their doctrine de facto.
I encourage anyone who wants to explain to me how Gay Marriage passed the Supreme Court in a different perspective.