DEAR ABBY: I am a gay male. My sister, “Cate,” is in what appears outwardly to be a committed relationship, but I know for a fact it isn’t. Her boyfriend, “Darren,” invited me out for coffee a few days ago, and while he was discussing some of their relationship troubles, he told me he isn’t sure that he’s straight. He then proceeded to say he could easily see himself dating me! I don’t know what t…