Are we supposed to believe she had sex with her date?

No. That was her implication to make her father upset, but she didn't do anything with her date; she had him take her home from the party early so she'd be home in time for her curfew.

Too bad she didn't come back as a talking duck. Then at least her character would have been more interesting.

At least she was more interesting than Buzz, thanks in part to the "Three Phases of Sonja" she went through (1. spaced out, 2. tomboy, 3. sophisitcation).

always found her more interesting then buzz he was kind of dull.