Both deception and coercion violate Kant's principle that it is morally wrong to treat a person merely as a means. Through deception when lying or withholding information one person is affecting the other's ability to reason properly. This is an immoral act and clearly uses the person merely as a means. Coercing someone into having sex violates Kant's principal by eliminating the other persons voluntary consent. When a threat or force is used the victim is put in a position where they can not reason and make a decision of there own will. By eliminating the other persons voluntary action the perpetrator effectively uses the person merely as a means violating Kant's principal.
One of the most common instances to use someone merely as a means for sex is through deception. The "I love you" line seems to be the key to the chastity belt (at least in the movies). By deceiving the person through a lie you are inhibiting their ability to reason properly which directly violates Kant's principal. This act becomes immoral when the other person would come to a different conclusion if provided all the information.