Initially, Marissa just wanted to prove a point. She wanted to prove to herself that she was attractive and irresistible, and that it was Emerson's loss for not reciprocating her feelings, but somehow things got out of hand and she ended up sleeping with Danson. She was so lost in the moment that everything just happened naturally.
But the thing that surprised her the most, was the memories that slowly materialized in her mind. Memories from 10 years ago.
For 10 years, she thought Danson raped her while she was drunk and took advantage of her, but while she was immersed in the pleasure of the man's touch last night, she began to remember how she felt during that drunken night 10 years ago. Not only did she kiss him first and seduce him, she enjoyed sleeping with him that night too. In fact, she remembered climbing on top of him, clinging to him and not letting him leave her body even when he tried to resist.
If anything, it was more like she raped him!