Let's face it, any peaceful alien race probably has something like the prime directive in star trek and will go to extreme measures to hide their presence until we're developed enough to make contact. They've probably observed us enough to notice we're pretty much a warrior race or that we've got a majority that homophobic or discriminating against our own race on appearance and figured most of us would become xenophobic and they should stay the eff away.
To assume that aliens, if they exist, think like man, I can forgive, since it's hard to imagine another way of thinking even if an alien way to think would be more realistic. However, your assumption that they think like a small group of humans that only have existed for a small part of human history (that I assume you are a part of) makes me think you should expand you horizons a bit. The potential aliens might just as well reason like this: "They're not usable as gladiators for hire to entertain us, even if we pay each winning gladiator with the knowledge of a technology that is decades ahead of their level, as they have to many inhibitions to fight in an entertaining way. They're not advanced enough to make porn extremely superior to the real thing (something that wiped out all of our specie except our ancestors, blessed be their wisdom, that saw the evil of unproductive sex). Even without this temptation they not only allow unproductive sex but in many places encourage it, even between individuals of the same sex. Since many of them are offended by the idea of superior races of their own specie how will they react to our superiority? Better stay away." (Note that this alien still is peace full. It won't attack or force amyone to do anything. The gladiators are volunteers that go for their own reasons, like the hope to become rich from alien technology when they return victorious)