It symbolizes our moral dependency on another person and protection of our body. Untying a persons strap (opposite sex) symbolizes that we will get their favors. If we break the strap or it is taken from us, then it represents a loss of authority.
Dream dictionary: strap dream meanings
(See Bond’)…
…(Band | Chain | Fetters | Shackle | Strap | Ties) In a dream, a bond signifies assiduity, piety, a pretext, an illness, an ongoing debt, cognizance of the one who puts the chain, or a rope around one’s hands. A silver band in a dream means a lasting marriage. If it is a brass band in the dream, then it means decisiveness but with trickery and regarding something loathsome. If the bond is made from lead in the dream, then it means having determination about something weak or unworthy of such attention. A bond made with a rope in the dream signifies piety and religiousness. If it is mad of wood in the dream, then it means persistence in hypocrisy. If it is a bond made of a piece of cloth, or a thread in the dream, then it means attachment to something that will not last. In…