I love this one!

Wam Bam Thank-You, Spaceman (1975) : Because their planet will be destroyed in 500 years, two goofy-looking aliens land on Earth and commence "Operation Procreation" with a stroll down Hollywood Boulevard. Liking what they see, the two extraterrestrials do what they do best, abduct Earth women for some quick, cheap flying saucer sex! Among the oddball side effects: the women are returned to Earth transformed into sexual dynamos and nine months later, a baby alien is born.