Question

________ remains the key factor that distinguishes a sexual assault from an unpleasant sexual experience with an overly aggressive partner.

Answer 1

Lack of consent.

Sexual assault is completely unwanted and non-consenting, whereas an unpleasant experience was consented, just not quite how you planned it.

Example:
Sexual assault: "No, no, stop."
Unpleasant experience: "Well, that wasn't as good as I expected."