I've heard this argument before and, sorry, but it doesn't hold up either; as you've said Paul's injuction against homosexuality was meant for members of the Christian community, not for the world at large.
I see then that you've abandoned the meaningless accusation that the Christian in question is not following the Bible with regard to shaving, breaking the Sabbath, and etc from the Old Testament and moved on to New Testament questions. This is a slight progression.
Paul is indeed talking about the world at large in Romans 1, when he characterizes homosexuality in the Gentile world (UNBELIEVERS) as a punishment for their rebellion against God. In 1 Cor 6, homosexuality inside the church is condemned as an ongoing rebellion within the church, a sin that keeps people from participating in the kingdom of God.
Christians who would like to affirm homosexuality need to reconstruct Pauline Christianity a bit.
He was saying, "Don't do this" not "Go out and make sure noone else is doing this".
Using Paul to justify this sort of thing is still playing fast and lose with interpretations.
There is no injunction in the Bible for Christians to prevent non-believers from participating in homosexual acts, but since heterosexuality is part of Christian theology (eg., the unification of Christ (male) and the church (female)) and is part of Paul's teaching (that homosexuality is rebellion against God, many Christians feel that homosexuality should be prevented in society to prevent God's judgment.
It's not playing hard and loose at all with Paul or the New Testament. Paul is explicitly against homosexuality, and that was never questioned until Christians wanted to affirm it in the post-modern age. To follow Paul literally is to be against it as he was, but as I have argued many times, this goes against Christian compassion and love in our modern era.