About as natural as you can get are animals, and scientists have found homosexual animals. They have no outside influence on them, making them chose the same gender rather than the opposite. Therefore it is illogical to say homosexuality goes against nature, when things found in nature are sometimes homosexual.
http://en.wikipedia.org/wiki/Homosexuality_in_animals
The only way you can attempt to prove homosexuality to be natural through the animal kingdom is by giving examples from animals that do not have sex for the sake of plesaur.
Animals that have sex for fun, such as monkeys showing homosexual behavior, can be compared to as humans homosexuals, since they both have sex for fun. Therefor this is not somethign that you can prove to backup your statement.
Animals that have sex for survivel of the specie, show homosexual/bisexual behavior. But you need to ask yourself why it is that they have sex with their own gender. Its not like they like having sex and prefere their own gender to have it with, cuz that would suddenly make them animals that have sex for the sake of pleasur, of which they do not!
There MUST be a logical explanation for why they show homosexual behavior.
My theory is that they need to get their semen out somehow and are thereby forced to have sex with their own gender! In the animal kingdom females are not in for sex 24/7! They are in for sex a few days, weeks a year and thats it. Now what is this male animal going to do between the females sexualy active periods? The semen production continues, and he needs to get it out one way or another! It cannot masturbate!
And therefor it is forced to have sex with his own gender!
So the animal kingdom cannot be used to backup your statement stating that homosexuality is natural. I already discussed this with schredder, so if you'd like to you can go a few pages back and read.