Something is queerbait if they make characters seem gay but turn out to be straight. Euphonium is queerbait because of all the KumiRei stuff while making sure to occasionally remind viewers that Reina is in love with her male teacher. I wouldn't consider things like Madoka Magica and Love Live to be queerbait because they don't do this, even if they don't 100% canonize any gay relationships.
Like AkiraH, I'd prefer it to be honest het to queerbait. Yuri is my jam, but I can enjoy good het, too, and there are plenty of other things about Euphonium to like.
Also, I love Love Lab.