Homosexuality is never condemned in scripture as an orientation, however homosexual acts are condemned as sinful. This is a divinely revealed biblical truth, such that we Catholics - despite our great love and sympathy for people afflicted with passionate homosexual urges - cannot but condemn it.
However, I really do not think that this particular passage refers to adult, male-on-male or female-on-female relationships. I cannot lie. This idea is actually ludicrous if you consider the context.
First of all "arsen" refers to "men", masculinity so it limits the scope of the passage to male sexual deviancy. Lesbianism is thus not within the scope of this passage, whatever it means it is restricted to men.
Basically: it refers to some kind of obscene, sexually depraved and perverted behaviour comitted solely by adult men for which there existed no adequate word in the Greek language, such that Paul had to basically invent a whole new word so as to be able to convey the meaning he desired to express. That's all we REALLY know for certain.
Although the word in English Bibles is interpreted as referring to homosexuals, we can be fairly certain that this is not the meaning that Paul wanted to convey. If he had, he would have used the word "paiderasste." That was the standard Greek term at the time for sexual activity between males - it was just as common as our modern term, "homosexual". If Paul had thus wanted to speak about "homosexual sex" then he would simply have used this word, which he obviously would have known about and so his meaning would be clear.
Paul however, never did this. Instead he actually coined a new word - obvously attempting to condemn some obscene, sexually perverted act committed by men for which there was no specific word in the Greek language.
I actually think that he was condemning paedophilia and child-abuse.
Many would consider "malakoi" -- the word preceding "arsenokoitai," in 1 Corinthians -- to refer to a catamite: a boy or young male who engaged in sexual activities with men. Such boys were often slaves, owned by rich men as sex partners. This was a truly obscene form of "pederasty", which was rampant throughout the Roman world. Thousands upon thousands of innocent young boys sold into virtual sex-slavery so that they could be abused by lustful older men. It was horrendous.
The second term probably refers then to the men who engaged in sex with the catamite boys. That is, they were abusive paedophiles. The New American Bible contains a footnote which reads:
"The Greek word translated as 'boy prostitutes' [in 1 Cor. 6:9] designated catamites, i.e. boys or young men who were kept for purposes of prostitution, a practice not uncommon in the Greco-Roman world....The term translated 'practicing homosexuals' refers to adult males who indulged in homosexual practices with such boys."
Harper's Bible Commentary (1998) states that the passage refers to:
"... both the effeminate male prostitute and his partner who hires him to satisfy sexual needs. The two terms used here for homosexuality... specify a special form of pederasty that was generally disapproved of in Greco-Roman and Jewish Literature."
Given the context this appears to me to be the most appropriate explanation.
I can not understand why Paul would go to such lengths, grappling with words to create a new term that as yet did not exist in the Greek language, if the meaning he was so desperately trying to convey was simply adult homosexual sex, for which there already existed a common, everyday word that everybody recognised.
This is just implausible I'm afraid.
Honestly, we will never truly know what these words mean as they are basically impossible to translate, however I think the context favours most of all child-abuse through the sexually enslavement of young boys as "calamites" - boy prostitutes who were abused by older men.
Its my firm conviction then that the central portion of 6:9 might be accurately translated as:
"male child abusers and the boys that they sexually abuse."
This kind of act