The angels mainly took the forms of humans to:
A) make it easier for the humans seeing them and
B) easier for readers.
It's way too late to be flicking through the books looking for quotes, so I'm afraid I'm gonna have to state all of the above with no actual proof... if anyone knows the passage by heart... or has the book handy, you're more than welcome to quote it...
They epitomise PLATONIC love
(http://www.dictionary.com...)
Transcending physical desire and tending toward the purely spiritual or ideal
Just because they take the form of men (buff men, albeit, didn't help...) and are deeply in love, doesn't mean they are gay. It means they are amorphous, spiritual entities who share a special bond and just happened to choose the form of humans.
Rargh!