Because the Constitution guarantees the right of gun ownership, but does not guarantee gay marriage or smoking pot. Why is it so hard for liberals to understand that the Constitution is the law of the land? If you want to change the Constitution then go for it - elect representatives who agree with you and then they can try to pass an amendment to the Constitution. It's called following the rules.
Your argument shows a lack of maturity, knowledge of how our government works, and understanding that your need for others to respect your desires is not an issue.