I used Craigslist for about 8 years to meet "curious" guys. I think that my cock was the first (and often only) cock that about 25 guys sucked. I liked to meet first timers because they were usually attached and probably not going to have unsafe habits. I live in oil country in Alberta, Canada and I'm amazed at who sucks cocks. A biker, a heavy duty mechanic, a cop, LOTS of big burly oil workers and just "normal" looking guys.
In almost every case they were curious for 2-5 years before they actually met me. One guy was curious for 20 years!!! He was one happy camper when my cock got into his mouth!

They're not straight then bi-curious at the very least..

Show me a man who says they've never thought about it and I'll show you a liar. Same goes for women. There's not a human being alive that hasnt had a curious thought at least once in their lives.

You have put your finger on a gigantic cultural change, up-for-it. I suspect that in the past, such 'tabu' fantasies came up but were promptly stomped out because they were so threatening to anyone having them. They were just too scary to 'dwell' on.. because they "could only lead to a life of trouble."
These last few years of the internet have really opened people's eyes to so that much they never even dared to consider as possibilities before.

As an older man, I can tell you that homosexual exploration.. even vague 'interest' was considered such a potent tabu in the past, that few men would have had the guts to tell even one close friend about a **** desire that everyone had always told each other was just too 'sick' to even contemplate.

Suddenly there's this way for EVERYBODY! to communicate with complete strangers, and some guys start seeing that it's ok to admit that, yeah, seeing other men's erections isn't so 'disgusting' after all, in fact, it 'can' give them kind of a chub 'sometimes'.. "even though I'm 100% straight and would never want to touch one for real. And of course not put my lips on one!"..

As this attitude spreads, guys start realizing that they aren't at all alone with their 'unspeakable' fantasies. –That lots of other men have the same ones.. Faster and faster, a formerly fortress-like wall of silence starts breaking down. The breaking down snowballs with increased communication.. all between 100% safe, anonymous strangers who can never 'taddle' on them or punch them in the face or kick them in the dirt. (Our fears, all our lives, are still rooted in our horrible childhood humiliations.) Pretty soon we see, hey! seems like a LOT of guys like their own dick, seeing how it looks hard, etc. And now even showing everyone to admire it!

So everyone slowly realizes that, hey, it's hardly a stretch, then, to see that a guy can also admit that seeing another man's dick get hard makes his own get hard, too! Just like seeing another man slurping a juicy chicken bone can make your mouth water, too! It's all a process of stepping further away from the fear of being scorned and hurt by others.. once we accept that none of us are really that different.. and that homo vs hetero attraction is truly a sliding scale, not black or white, and that seeing sex is just plain infectious! It immediately makes you want some, too.. and makes your body ready for it! Who could know any of this while we were all still so petrified to communicate with each other about practically everything turning us on right and left? This cultural shift ...an increased understanding of ourselves, is one of the truly magnificent things to come from the internet.