It was a combination of things. For one thing, I've always been a bit bi-curious and I'd fooled around (and somewhat enjoyed) with a couple of guys in college daze (long, long, long ago.)
Then came a time when I was hooking up every couple/few months with babes (mostly MILF types for me) from here, eros, RB, etc. and I ran into a really attractive MILF who offered GFE and/or heavy BDSM. (Found the old review from back in '08 - provider ID = 138109.) Among the many very kinky things we got into, I agreed to try some strap-on stuff. I had somewhat mixed reactions but overall in the Heavy Kink scene it was pretty hot.
I continued to see cis-ladies, and many hot, hot, hot ones, but the strap-on experience lurked in my mind.
Then also the volume of online porn was building, and I stumbled into a porn video of a very hot trans lady just stroking herself. By my 'standards' today that video was rather mediocre, but it really triggered my curiosity, and I started wondering what it would be like to have actual dick in me. (Even in the college days, I'd never taken penetration, was more interested in being the "top" in those early experiments.)
Anyway, I finally took the plunge. The first experience was so weird and startling to me that I left the date after something like 15 minutes - only a couple/few minutes of actually getting fucked. I just kept adding on new experiences.
Today, most of my hobbying is for Trans Ladies. There's just SOMETHING about a hot woman coming at me with a Hard Dick that's an immense turn on for me.