Because the Greek culture I was referring to is Classical Greece. Paul would've come about 500 years after that, who while born in the Mediterranean area, would've lived with Roman culture. Already within the Roman Empire there is subtle changes to same sex relationships, the Roman Empire is heavily steeped in tradition, much of what dictated what was socially acceptable was based on perceived honor, virtue, liberty, and family. As such male-on-male sexual relationships were fine within certain stipulations. A Freeborn man could have sex with another man if 1) The free-born man was penetrating and 2) the other man was of a "lower class", i.e a slave, a prostitute, etc. Any one else and it would be constitute a loss of status for the one taking, and in Roman society it can't be overstated how important status was. Of course this isn't to say all homosexual relations were purely based on power or that there couldn't be a romantic/sexual relationship between men of equal class, but most as perceived within the society followed traditional views.

With Paul again he's already several hundred years past the point I was intially talking about, and of course just because someone lives in a specific area/culture doesn't mean that they can't hold ideas of their own differing from said culture. However the empires stance on same sex relationships would of course change as it gradually became more Christian before being a Christian empire, with male prostitution being outlawed not long after Paul died, anal penetrating would result at being burned, etc but this is rambling from the point. The earliest times we see this change on same sex relationships is with the first Christian polemics (such as Paul) who preached on the vices of Rome decadence with a core critique being same sex relationships as a "sin". Christianity as a whole was considered radical for the time given the Roman culture so it's not really a surprise that Paul would similarly hold a different view of same sex relationships then what was then accepted.