The good old "we don't serve gays" argument.

The baker case I think was about the cake, thus having to produce "reprehensible" art. Editorial work (steam selecting what games to promote) is Sth I strongly think is their right to choose. But having a transaction run through your network? That's on the other side of the spectrum, way closer to the "no gay customer" situation. If they'd at least pretend it's because of, idk, higher card fraud rates...