Interesting concept for a story, the parasyte takes over the body of a sex crazed woman, takes advantage of the fact she's very attractive to easily lure men (and women) with the promise of sex, but the parasyte itself is still a virgin. Realizing her body (and maybe some leftover parts of the human brain structure she copied over) are craving for sex, she starts to try it out and gets more and more into it, being able to user her "powers" to enhance the pleasure she feels from it.
In the end, she starts having sex with her prey like a normal woman, but during sex she starts changing shape, growing more mounts and other parts of her body. While the prey starts to freak out, she continues to rape them, mixing fear, pain and pleasure to get more and more powerful orgasms out of the, until she's finally done and can eat them.