1) Roddenberry wanted to suggest everyone is open-minded and prejudice-free about sex in the future, but sometimes this point didn't come across. Other times it come as just weird.
2) One word: Kirk
3) Supposedly Vger was travelling too fast for the Klingons to pursue and for the other staships come back to Earth in time
4) torpedoes offscreen, E-9 took more time to assimilate because it's much larger than spaceships
5) Sci-Fi Writers Have No Sense of Scale
6) see 5 above, also applies