What is "sex"? What is it's purpose, what's it for, how does it work, why does it even exist?
First question difficult to answer as different people tend to define having sex differently.
As for purpose/what it's for, this can range in any number of things. In no particular order and not necessarily focusing on just humans, pleasure, social bonding, to express love, establishing dominance, reproduction, stress release, exercise, compensation. Not all have to apply, just one will suffice.
How it works would depend on how you're defining it.
Likely arose because it diversifies genetics more efficiently than asexual reproduction. But as noted, that's not it's only function.
What is "love? Is love sex? Is love part of sex, or is sex part of love? Does love need sex? Can two people love one another with sex? Can two people have sex together without love? What is first, or prior, or more important? Which should lead the way in relationships? Which should be the master, and which should be subservient - between sex and love?
Q1: A series of chemical reactions resulting in various emotional states.
Q2: No
Q3: No, but can be used as a form of expressing love.
Q4: No
Q5: Yes
Q6: Yes
Q7: Depends on the person.
Q8: Again, depends on the person.
Q9: Neither
What is "Nature" and is the most natural way necessarily the best way in all things?
No it's not, in fact such a statement is call the naturalistic fallacy. The response of pointing out that homosexuality is natural is to counter the argument that it isn't, as if it not being natural some how makes it wrong. Which in itself is committing the naturalistic fallacy.
What is "choice"? How do you know when you have a choice and when you don't?
think wiki states it best.
"Choice consists of the mental process of judging the merits of multiple options and selecting one or more of them. While a choice can be made between imagined options ("what would I do if ...?"
, often a choice is made between real options and followed by the corresponding action." -wiki
"An option; a decision; an opportunity to choose or select something." -wiktionary
You weren't actually wanting these answered were you?