Addressing first question: In my own opinion, I do not believe, never will believe that homosexuality is a choice. People who believe that being gay is a choice chosen by someone, they're ignorant, arrogant and self-righteous. Therefore, based on this belief, they should have just as many rights as the rest of us. You can quite easily draw parallels from the slavery era and the current-day rights of homosexual. Nearly every person on earth today can agree to the slavery era as being wrong, but then some turn and protest the rights of another unchangeable characteristic of a person, and so I think that with time, homosexuality will be just as accepted as African American folk. It'll just take time for society's acceptance, but is in no way wrong in my own belief.
Addressing the second question: I've actually debated this question before in a class. People who are against gay marriage are almost always dabbling, have dabbled or have full belief in the bible, and believe the rules the bible sets. However, disowning this thought because truly there is no argument that can be made in reference to biblical references because they're so undefined and can be easily translated in more view then one. And not all people believe in the bible as being a 'true' book, and believe the bible is not as real as religion holds it up to be.. So, after asking people in class to disregard all evidence originating from the bible, I noticed many arguments were nulled.
A second obligation to gay marriage that has been made is the separation of church and government, the government has a foot in the door of marriage whether you choose to acknowledge it or not. Marriage is a legal process, though some dispute this to be more a spiritual then legal. Which I can agree to both, but since marriage is a legal process and The Declaration of Indepence clearly states 'We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights' -- Piecing this out, "all men are created equal" should be something that is practiced whether you are homosexual or not, by god I don't know how people still protest that. People often use the 'their Creator with certain unalienable rights.' Creator referring to god, I assume which infact the god the bible depicts tells us to love all and treat eachother with equality. Wether they are gay, black, fat, skinny. Love your neighbor. This is something that is often forgotten.
I may or may not have mixed my words in this, I just ranted about. Questions can be clarified.