All forms of sex are choice.

If you are not being raped then you made several choices to be in bed with the person you're in bed with. The argument of is a thing natural or unnatural is purely irrelevant because everything but rape is choice. People spend their time defending their actions by using the word nature while others attack their actions with the word choice. This is all based on personal desire of the individual. We don't realize how much our beliefs & view of ourselves causes us to be attracted to someone else. We may say I like redheads or fat chicks or men with big butts, but all of this deals with an inner sub conscious thought process that we have built up for ourselves throughout the years.  It only is natural to us because of how accustomed we are to this thing we created, or sometimes allowed society to create for us.
So is anything natural by this definition? Yes, humans natural go to where they find success. Most poor or average folks would never try to date a famous person because they feel that they have no chance. The many disappointments we have in life very often guide us into a direction that makes us feel that we are naturally attracted to someone or something. Sometimes we are attracted & sometimes we're not attracted to what we've grown to believe we're attracted to. We often try to solve a spiritual desire (sex) with physical actions. Most of us don't even know how to properly, mentally & spiritually use sex to liberate themselves instead of having sex create more bondages.
In the essence what we are saying is it's all forms of attraction & sexuality are both choice & natural. If a human is attracted to the opposite sex, the same sex, animals, dead bodies, food, children, machines, urine, dodo, vomit or anything else, it is all under the same umbrella of natural desire. It is also completely natural for them to make the choice to attempt to fulfill those desires. If they are successful in their attempts, it's natural to continue in like manner.
So the next issue is that we have found very often personal preference leads to judgment. For example since I am attracted only to women, & you're attracted to only men you are wrong & disgusting. What I am attracted to is natural, while you're a pervert for being attracted to something else. Then we come up with reasons why you're wrong for doing the same thing that I do, but with what you are attracted to. Another example is she is a lesbian, but she judges pedophiles & necrophiliacs. Again, saying that what I do is right & what you do is wrong.  
Many of us view age & the ability to consent to sex as being the same thing. This has nothing to do with one's maturity or mental stability. There are many adults who don't have the mental capacity to consent There are many adults who are idiots, even furthermore there are old people who are fools. They are 70+ years old yet still have not "removed childish things". In other countries & historical backgrounds our views of who to marry, love & embrace are different. America has a large amount of restrictions. This is why we're so shocked when we see nudity but people in other countries don't see it as a big deal. So are we defending or attacking any form of sexual desire? No, because we do realize that there is nothing "perverted" about them. They're all natural acts. If a person has sex with a machine or a piece of food & enjoys it who am I to judge. Many people enjoy getting pissed, crapped or vomited on, are they wrong for doing what gives them success? No.
The only thing that's wrong is when we start judging others for being attracted to what we are not attracted to. Then we get offended when we are called wrong or unnatural or perverts or disgusting. It is time to look in the mirror & stop looking at what others are doing.