In the historical context in which Paul wrote his epistles, sexual orientation as a concept didn't really exist. There was no real conception of monogamous same-sex relationships. In Roman society at the time there wasn't any particular taboo against men sleeping with men. What was a major issue on who was doing the penetration. It was considered unmanly to bottom. So when Paul wrote his letter at the time, it is more probable he was addressing those early Roman Christians at the time who lived in a society where it was socially acceptable for them to rape their male slaves for fun.

To extrapolate from Paul's condemnation of "men who sleep with men" a ban on healthy, monogamous same-sex marriages is something he wouldn't have had in his mind at all when he wrote it because it just wasn't even considered.