Casual sex has nothing objectifying about it. Some men objectify women through the way they practice casual sex (i.e. trying to rack up a score), but that's an entirely different matter that has next to nothing to do with casual sex itself. It's like how some people use meds on their kids because they don't know how to be parents; it's not the meds that are the problem, it's the parents. Likewise, it's the douchebags who objectify, not the sex; even without casual sex, women are still constantly objectified all over the place.
It may or may not be that a majority of men practice casual sex in a score-keeping, trick-her-into-bed, objectifying fashion, but that's a commentary about the culture surrounding casual sex, and not casual sex itself, which is just people having sex outside the framework of a relationship. That's a vague phenomena that can be assigned multiple meanings. It's a cultural problem, not something inherent to the act of casual sex.
EDIT: It's worth noting that by "objectification" I understand the notion of using a person as a means to one's own ends in open disregard to their well-being, subjective experience or goals. There is a wider understanding of objectification that includes, for example, objectifying waiters and waitresses because we don't seriously care about them as people, but I'm talking about the narrower notion of objectification that simultaneously dehumanizes and degrades (i.e. denies the subjectivity of the other, rather than just ignoring it or treating it as inconsequential, as seen in benign objectification).