It is heavily implied that Marcia's best friend from school, Noreen, is a lesbian. The girl clearly has a crush on Marcia, and they sleep in the same bed while having a sleepover, with only their heads showing. Everything else is covered by blankets. Noreen even rubs her leg against Marcia's under the covers. Of course, throughout the movie, Marcia is completely unaware of Noreen's feelings for her, highlighting the naivety of the entire Brady family.
Marcia rejects her date's sexual advances while they are sitting in a car. She says that she's not the kind of girl who goes to third base, and as a result, he makes her get out of the car.
When Marcia is afraid her date won't want to take her to the dance because of her injured nose, he says "Don't worry, it's not your nose I'm after". Just a minute later, when Mrs. Brady asks him if he has protection against the rain he says, "Sure I do", and describes having assorted colors and textures. Both lines have sexual implications, although it flies right over Marcia's head, as well as her parents. They obviously don't get it.
Cindy walks in on her parents in the middle of the night, presumably while they are having sex. No nudity is shown, however; we just see them in bed, with Mr. Brady on top of Mrs. Brady.
A female neighbor of the Bradys' says to Cindy, "Tell your father I'd be happy to wrap his packages next Christmas." This obviously has sexual undertones, but Cindy happily agrees to deliver the message to her dad, and then she does. Mr. and Mrs. Brady don't get it, either.
The same female neighbor of the Bradys' tells Greg when he comes over, "You sure have been getting bigger" and "You're almost as big as your dad"; both could be sexual innuendos.