Maybe because they combine our images of the rural Adonis -- barbaric, uncivilized, passionate, sexually uninhibited -- with our image of the religious -- inhibited, virginal, other-worldly.
Or maybe it's the cute costumes.
There are about 250,000 Old Order Amish in the United States, with concentrations in northern Indiana and western Pennsylvania. They are followers of an Anabaptist sect, with beliefs similar to today's Baptists, except for their separatism: do not be influenced by the "evil" world outside. About half speak Pennsylvania German, a dialect of Palatine German. Most refuse automobiles, television, or the internet, although they will use mechanical devices. I can relate: when I was growing up in the Nazarene Church, we had cars and electrical devices, but we were forbidden "worldy' costumes, dancing, movies, theaters, circuses, carnivals, rock music, mixed swimming, alcohol, shopping in stores that sold alcohol, etc.
It's not all plowing and Rumspringa. Amish kids are not educated past the eighth grade, so they tend to lack awareness of science, environmental issues, cultural issues.
The church is vehemently anti-gay. Marriage and reproduction is a strict requirement (six kids on the average). Gay Amish kids who come out to their parents are instantly banished from the community.
I've never met a gay ex-Amish guy, that I know of. I imagine that those who leave get a hair cut and change clothes.
I've never seen a real Amish guy with his shirt off, either, but I imagine the view is something like this.