>>3164A bit over a year ago I was watching Gao Gai Gar at the same time as reading EE Smith's Skylark series, and it occured to me that Skylark was formulaicly similar to most of the super robot animes I've seen. You've got your existential extraterrestrial threat, and humanity is in an arms race with it til the action reaches outrageously over-the-top levels. Smith's other big series, Lensman, also fits this formula, and the nips actually did make a couple anime adaptations of it.
These are pretty old books so the girls might not be quite as lewd as you'd like, though there are many planets where nobody wears any clothes, including one populated entirely by amazons and another where it's normal to spank your wife. The main girls are involved more in a romcom type plot than a harem or moeshit one though.