1. When was the last time she got laid?

Not as recently as she would have liked.

In SP, Batman and the justice system have cockblocked her and the Joker with the whole committing them to an asylum thing.

In C&C, Joker has cockblocked her and Andy with his menacing clowny presence or something.

2. What was my character's first time?

Diving off into utter headcanon-land, Harley's first time was probably early on in high school with her first ~real~ boyfriend.

He was older than her.

And when they broke up she became 1000x more concentrated on gymnastics and it actually helped her earn that scholarship to Gotham U.