Exchanged (3337) (metallasso from metá = change of place or condition + allásso = change) is used only in this verse and the preceding in the NT and means to cease one activity and to start something else in exchange. It denotes the giving up of one thing (the natural function of the woman, heterosexuality and child bearing) in order to receive another (the lie of lesbianism). Women became lesbians, practicing unnatural sex and knowing no shame. The verb exchanged is aorist tense (action at a moment in time) and active voice (subject makes a choice of their will to carry out this action) indicating that these women made a deliberate choice to exchange the natural for the unnatural, which was a reflection of the influence of the degrading passions God had given them into the power of. Expositor's Bible Commentary observes that... Paul's use of "exchanged" is suggestive. The first exchange, that of the truth for the lie, is followed by another--the upsetting of the normal course of nature in sexual relations. Instead of using the ordinary terms for men and women, Paul substitutes "males" and "females." The irony is that this sort of bestiality finds no counterpart in the animal kingdom. Perversion is the unique contrivance of the human species. (Gaebelein, F, et al: Expositor's Bible Commentary: Old and New Testament 12 Volumes)