LIBERTARIAN PARTY PLATFORM 1:23
We advocate an end to all government attempts to dictate, prohibit, control or encourage any private lifestyle, living arrangement or contractual relationship.
We would repeal existing laws and policies intended to condemn, affirm, encourage or deny sexual lifestyles, or any set of attitudes about such lifestyles.
And section 1:3
We advocate... the cessation of state oppression and harassment of homosexual men and women, that they, at last, be accorded their full rights as individuals.
It's right there in plain english.
First off, the platform you cite doesn't address the Constitution.
If you disagree, please cite the areas of the Constitution involved.
Second, even if their platform did wish to change the Constitution, if they advocate doing it in legal fashion there is no usurpation at all.
Legally changing laws is NOT usurpation.
Therefore, as many of us saw from your first post, you don't know what you are talking about.