I'd forgive him if it were a matter of failing to carry Old Testament proscriptions and prescriptions into the current age. St. Paul was rather definite on the idea that belief in Christ means freedom from the Law, and highly critical of those who asserted that adherence to the Mosaic Law was a necessary part of Christianity. In a fit of exasperation in Galatians 5, Paul says of those who assert that circumcision is necessary for salvation, "As for those agitators, I wish they would go the whole way and emasculate themselves!"
So, it's quite possible to square a belief in literal interpretation with discarding Mosaic law.
... the proscription of homosexuality is part of the Mosaic law, ne? So if eating shellfish is no longer an "abomination" then maybe being gay isn't either!
I don't think there's anything in the New Improved Testament that addresses the subject.
The short answer is: There is some New Testament stuff to deal with, not to mention a general proscription against "sexual immorality" that Paul found particularly serious (but not serious enough to define with any particular precision).
stePH is quite right---most of the biblical stuff about homosexuality is in the Old Testament, and when somebody uses those verses to condemn your choice of bed partner you can quite rightly point out that they belong to the Mosaic Law as much as the dietary prohibitions do. Now this gets complicated, because there are some pretty reasonable laws in there too. For example, Leviticus 18 contains prohibitions of sex between males, but Leviticus 19 (which is part of the same set of rules) contains prohibitions against stealing and the sexual assault of slaves.
Of course, those sorts of things are covered equally well in the New Testament. Jesus reinterpreted and simplified the Law to quite a radical degree. In the Sermon on the Mount, for example, we learn that adultery is just a symptom of deeper problems and hatred is equal to murder. In every case the problem lies in the spiritual state of the person breaking the Law---intention and motivation are far more important to Jesus than the actual sin itself.
Stealing isn't wrong so much as the desire to possess what isn't yours. Sexual assault isn't so wrong as the need and desire to dominate.
Paul's take on this is that we are free from the Law, although we have to be careful not to act in a way that could harm the consciences of others. Sometimes you have to respect the hang-ups other people might have.
So when he gives lists of behaviours that are against the spirit of Christianity we've got to listen, but we also have to be careful we're not reading things he didn't intend to say. It's very easy to simply read, 'Oh! He's railing against the behaviour I most like to hate!' Generally speaking, that's exactly the message we shouldn't be getting.
For example, in Romans 1 he lists a whole lot of things that are generally considered to be quite off morally. It's very easy to be smug and self-righteous when reading this. It includes things like idol worship, homosexuality and general nastiness. The kicker comes in Romans 2:1 when he says
Therefore you have no excuse, whoever you are, when you judge others; for in passing judgment on another you condemn yourself, because you, the judge, are doing the very same things.
He then goes on to point out that even the most moral of people do not achieve the perfection required by God. This is the basis of the 'Judge not lest ye be judged' saying, not to mention 'He who is without sin can cast the first stone.'
Does this mean the behaviours described are okay? Probably not, but it's hard to tell. This passage (and others like it) aren't there to rail against homosexuality but self-righteousness. Furthermore, what Paul is talking about seems to be different to what we would understand as 'homosexuality'. For example, Paul often seems to assume that men choose to be that way, and there are all sorts of questions about the nature of homosexual practice in that place and time.
The upshot of all this is that the Bible seems to take a dim view of homosexuality, but a close reading suggests that God mightn't have a problem with monogamous relationships. What the boys wanted to achieve in
Brokeback Mountain was a far cry from Sydney's Gay and Lesbian Mardi Gras.
Anyway, like I say to my congregation most Sundays, leave me out of it now. What you do is between you and God!