Discussions of the morality of homosexuality always go back to the authority of Scripture because, as Michael points out, it is consistent in both testaments on homosexual activity. The only way to biblically open the door to legitimizing homosexuality is to say that the biblical writers (esp. Paul in Romans 1) didn’t know what they were talking about as it relates to today. Therefore, that Paul and other biblical writers said about homosexuality is contextualized into ambiguity.
As to the argument you cited (we can paraphrase it as “that was then, this is now” TWTTIN), I too think this is one of the ways in which the argument for inclusion goes off the track. However an honest exegesis ought to at least explore how the social conditions diverge between our horizon and that of the text. That doesn’t mean that we discard the distant horizon, but at least to acknowledge the cultural distance and then test it as to how it applies now. The temptation would always seem to be to engage in a sort of anachronistic reading, an eisegesis that sees the present reality in the ancient text. That’s why we pay attention to the cultural background, not to get around a text but to escape the easy ratification of the Self. And just to be clear this danger of eisegesis comes in a raft of disguises from how we see questions of rich and poor to how we deal with marriage or sexuality.