I looked through the article in the link (actually clicked another link within that article) and the most detail I could find about that is in the following:
"Craig says Phillips “started to explain he had gay friends. And he would sell us cookies or cupcakes. But we left.”
Phillips recalls their anger. “They swore at me, flipped me off and stormed out,” he said."
From what I can tell, it appears that he wasn't going to make a cake for a same-sex event, but was still willing to sell them other things. So it's not like he was outright denying them service just because they were gay. It sounds like they then became unruly and decided to leave instead of buying anything.