When you are hungry, do you go and kill an animal and eat it raw, or open a packet and cook the contents?
When you need to poo, do you go and squat behind a bush, or go and sit on the lavatory and wipe afterwards, not forgetting to wash your hands.
When it gets dark at night, do you find a nice place to sleep, or switch the lights on and carry on with your day?
When you are ill, do you curl up and die, or go see a Doctor?
The point to this question, which unsurprisingly has escaped all the religious bigots here, is that these are all things that we all do every day, but they can only be observed in one species. Ours.

Homosexuality has been observed in many species. So what is natural?

I'm not gay personally, but if I was, it would probably be the most natural thing I've done today.