Homosexuality rules in the OT was for the LEVITES, the priestly class. They had a separate set of rules to live by apart from the other tribes.
The moral of the Lot story isn't about homosexuality, it's about hospitality, tending to the needs of guests (by offering up your own daughters for gang raping instead).
The reference about "men kept for unnatural purposes" is refering to catamites, or the nobilities' use of "boy lovers."
Context, religions...context!