I am speaking in human terms, because of your natural limitations. For just as you once presented your members as slaves to impurity and to lawlessness leading to more lawlessness, so now present your members as slaves to righteousness leading to sanctification. (ESV)
I love how Paul approaches his audience. He tells them he is using human terms because of their natural limitations. At first this sounds like condescension or arrogance. Paul was incredibly intelligent and much of what he understood would have been over the heads of many of the people who would have listened to him. Even the Apostle Peter spoke of Paul's intellect:
2 Peter 3:15–16
 And count the patience of our Lord as salvation, just as our beloved brother Paul also wrote to you according to the wisdom given him,  as he does in all his letters when he speaks in them of these matters. There are some things in them that are hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other Scriptures. (ESV)
So what is Paul simplifying for the Church at Rome? He is talking about our past life before Christ and our new life in Christ. While we may have thought that a life pursuing pleasures was a pursuit of freedom the very opposite was true. Impurity and lawlessness enslave us and they lead to even greater captivity. With each sin the shackles of slavery tighten.
Similarly, as we present our bodies as slaves to righteousness we grow closer and closer to what Christ has for us. He desires to mold us more and more in the image of himself. Our efforts to present ourselves to righteousness are not vain efforts to appease our Savior, but rather they are demonstrations of our cooperating with his perfect will.
Those who are in sin progressively become more enslaved. As believers in Jesus Christ we are to progressively grow in the process of sanctification. Surrender your will to the Lord and allow him to mold you more and more in his image.