Antinor01 covered the US Congress.
But when you say highest, that could mean State Constitutional Office, since they are elected statewide, while Members of Congress are only elected from a single Congressional District, which is only part of a state (usually).
Then the highest openly GLBT elected official would be Ed Flanagan, Vermont State Auditor.
While ministers aren’t elected, I know that the first same-sex marriage held in Norway involved the Minister of Finance and his partner, a doctor. I don’t know whether the minister was also a member of Parliament, as most Spanish ministers are.
Per-Kristian Foss was elected a member of Parliament, but once he became Minister of Finance he had to leave that seat. He is technically not “married”, but in a “registered partnership”. Apart from adoption issues and the right to a church ceremony in the state church, it is the same. (And I believe those two issues are starting to fall as well)