We discriminate all the time. We euthanise pets but not humans Why? Because we discriminate that cats are inferior to us (except for some crazy cat people). This is a good thing, and saves us money so we can treat more people.
We practice affirmative action in NZ, which is positive discrimination.
Manufacturers discriminate against left handed people all the time.
Airlines discriminate against very fat people by making them purchase 2 seats.
Gay people also discriminate when choosing their partner -- a gay man will avoid sex with a female -- he is discriminating against women.
Gay people are different -- this is not just a minor difference such as skin colour or height -- to the point, if everyone were gay, there would be no people at all.
You're right - and not all discrimination is bad, however for the state to widely discrimate against people for being something they are (and have no control over) is the wrong kind of discrimination. I cannot see how it is different from discrimnation based on race, religion, age, sex etc. All things that were heavily discrimated on in the past, but as a society we have evolved and recognise that this was wrong. My (future) children's generation will look upon this issue in a similar way to how I look at these other historic forms of discrimination - I can't believe we so openly discriminated against 'black' people and women until so recently - and even still now to some extent. Makes me sick.
Marriage in the tradition sense is between a man/woman, but this excludes gay people by definition. So civil unions were created to allow gay people to enjoy the same rights under law as a married couple. Yet, the gay people still don't have the exactly the same rights, as the laws of nature prohibit them from naturally bearing children. The gay people can argue that one with god :)
As I said previosly, it depends who's 'definition' you are refering to. If you are refering to the chuch's definition, it's irrelevant - the issue on the cards is the state's definition for the legal recognition of a marriage. What a religion recognises as a marriage does not have to change to match the law. The spiritual definition recognition of marriage is up to that religion.
The argument about children is also irrelevant. I know married couple both who cannot have children, or chose not to have children before they got married. If that is your argument, you should be arguing against all form of marriage where there is not commitment to attempt procreation - a totally different argument all together.