We're just very briefly going to look at the end of Romans 1 and the beginning of Romans 2. This is the transition between 1:18-32 (how all of us, with special emphasis on the Gentiles, are condemned) and 2:1-3:8 (how the moral majority, with special emphasis on the Jew, are especially condemned). He has just mentioned (in vv. 1:29-31) a long list of 21 sins that we all commit. Now Paul starts out the transition by saying
Though they know God's righteous decree that those who practice such things deserve to die
Isn't it interesting that everyone, atheist or not, knows this? Everyone knows that we deserve God's judgment on our sin. Paul adds one more thing to our list of things we cannot not know. It's a sad and scary thing when the threatened judgment of God no longer affects a person.
they not only do them but give approval to those who practice them.
Paul is saying that giving approval for these sins is in some ways even worse than doing the sins. Piper says, "Which is worse? Suicide or murder?" He likens our approval of things in society (like our society's rampant approval of homosexuality) to being spiritual and eternal Dr. Kevorkians.
Of course, at this point, some people in the audience are clearly not feeling condemned. Even if they were guilty of the 21 sins Paul mentioned in his vice list of vv. 29-31, they definitely didn't approve of the sins and often spoke out against them in society.
It seems to me that the reason for Paul mentioning how the Gentiles publically approve of these sins was mainly for rhetorical purposes to draw in the self-righteous crowd so that Paul could turn the argument to them. Watch how Paul continues:
Therefore you have no excuse, O man, every one of you who judges. For in passing judgment on another you condemn yourself, because you, the judge, practice the very same things.
Paul would have had a lot of experience preaching to self-righteous Jews during his missionary journeys. And when you travel around giving the same sermon, you learn what works and what doesn't work. You really notice what has an affect on the crowd. I imagine that Paul had used this trick a few times: first talk about how the Gentiles are approving of these sins. Then when the self-righteous religious person says "Not me! I don't approve of their sins", Paul uses this opportunity to start his condemnation of them.
And he, here at least, isn't condemning them for their hypocrisy (as is often stated), but rather he is showing that their own knowledge of the truth makes them inexcusable. (Paul has, ever since verse 18-19, been strongly emphasizing our own knowledge to point out God's justice in judging us. We actually just saw this again in v. 32 above.)
This little rhetorical trick reminds me of Paul Washer in his, now famous, "shocking sermon". I've tried to start the youtube video 5 minutes in.