Everything seems like a trigger these days; just seeing any kind of female does it for me. Someone walks past and I'm looking at her hair, face, shoulders, body shape, shoes, clothes, and just how comfortable and unaware of being female she is. Same for a picture in a magazine, or on television, or a female voice on the radio.
I'd agree with the hair as a huge trigger though, because it seems so far off for me and so unattainable, but every girl has great hair compared to me. Kinda like your beard issue, solexander; it's one of those things that cis-gender people just take for granted, even see as an inconvenience, not realizing that guys and girls like us would happily assume those characteristics in a second and never once complain for the rest of our lives.