A kiss is thought by a lot of scientists to be a way to exchange pheromones, amongst other things, or may serve more emotional purposes such as comfort, demonstrating trust, or forming closer bonds in general. Still seems to be up in the air as to whether it's a learned thing or instinctual, but signs are pointing to its relevance manifesting itself in similar ways across the animal kingdom (grooming, touching noses, etc.) and also somewhat corroborated by ~90% of the world's population engaging in it.
If you'd like to make the argument that physically speaking, your ass wasn't made to handle sex, you might also argue that neither is your mouth if you're sticking to the strictest sense of how each part functions, but I can speak to a lot of people who certainly enjoy oral sex ;P
However, most of this is irrelevant if we're to look at just what being 'natural' or 'unnatural' entails. If you'd like to define natural as something that just happens
with little to no outside interference, well, anal sex is just as natural as anything else. Human sexual urge in of itself is a natural thing, and so inventing new places to stick a penis for sexual gratification is hardly considered weird, especially when you consider that many straight men also have the desire to stick it up a woman's ass all the same.
The problem is that many people use "natural" as a synonym for "socially acceptable" or "commonplace", and they aren't quite the same thing. Cancer is natural, but we fight it aggressively. Farting is natural, but it's still considered rude. In some parts of the world, it's "natural" to belch after meals and find it complimentary.
tl;dr - there's always this.
In terms of your response, Jobe, I would just caution that you should be prepared to take questions on why you think homosexuality is a choice and perhaps present some evidence as to humans being 'born straight'. It's not a wildly popular or well-supported notion and you are in a community consisting of people with a wide range of sexualities.