Can someone explain how homosexuality is compatible with evolution?

Perhaps it would be easier if you considered how the non-reproductive ants continue to survive even though they don't reproduce. It's because their "sister", the queen ant, contains almost all of the same genetic material. When the non-reproductive ants support the queen, they assist in passing on their genetics to the next generation, though their sister. This is not unique to instects, and is common in any species that packs or functions as part of a community.... it's highly common in mammals. Humans reproduce less than ants, but they make up for it with high survival rates. Non-reproductive humans (males in particular) do not influence the rate of reproduction, but they do aide in helping their kin to survive and provide for their children, passing on their genetic material. Studies have been done examining how sisters of homosexuals are more fertile than average, perhaps as a biological consiquence of the genetics at play, or perhaps because the brother assists in the upbringing of her offspring.

Understand also, that evolution is not a one-way street towards "improvement". There are often tradeoffs, where the negative impact of an attribute are compensated for by other more possitive consiquenecs. The reproduction of a species and how it works in an evolutionary perspective must be understood as part of the whole system or community, it can't be understood just looking at the reproduction of each given organism or considering a narrow-sighted view of what is a helpful or hurtful characteristic. Evolution is complex and often not very intuitive.