The point is not that we will know everything someday; we probably won't. (Indeed, on a certain definition, we already know we can't, see also Uncertainty Principle, Halting Problem, etc.)

The point is that being unknowable is not a good thing. It's a very, very, bad thing in fact, because we can't control what we can't understand. If we never understand cancer, cancer will keep killing us. If we didn't understand astronomy, an asteroid could hit us at any time. If we never understand consciousness, we'll never invent AI.

(Also, your specific example is awful. We know that homosexuality is not unique to humans; in fact it is found in over a thousand species and counting. If it's not adaptive, it's got to be vestigial; and in fact it's probably adaptive. This is also morally irrelevant, but it's something we do in fact know.)