Most libertarians use natural law as the basis for their political philosophy. Many would oppose abortion for the same reason they oppose the death penalty. They do not believe that the state should ever have the power to take human life. And most libertarians would say that marriage is not a matter for the government to "license" one way or the other. So libertarians are not so different from many religious fundamentalists on these two issues.
It's a bit bogus to say that libertarian principles forbid the death penalty. The government can't execute someone. Okay, but why can the government lock a guy in prison for 50 years and control his private life? If the latter is extreme though okay, the first has to be, too.
I'm a libertarian. I know lots of other libertarians. We're all for the death penalty
The biggest main legitimate purpose of government is to deal with violent crime. As such, it needs the power to imprison and execute.