Actually the libertarians I know have no such beliefs they do however believe it is not the Government's job to determine who can marry who.
In fact they believe that the very reason there is contention over gay marriage is because the government took it upon itself to reward marriage with monetary goodies. (Income tax and Social Security Benefits) Libertarians believe that if the government had not done such there would be no legal grounds for Gays or Group Marriage advocates to petition the government for equal rights and being awarded monetary goodies for their version of marriage.
Quit the BS, libertarians are telling you their politics when they tell you that they are libertarian.
The libertarians exist/vote/run for office/speak/write/ participate in forums, to end/defeat political opposition to those positions, and to support/promote politics that support those positions.