Once people realise or they find out i'm in an long distance relationship (UK to USA - if your new to this place) they always wonder or think it's weird how I can trust someone I can't physically be with for most of the year. Apparently merely saying "I just do" is too hard to believe. Or understand.
Some people will never understand, or want to understand, and that's their prerogative. But and I'll borrow a line from my favourite band flyleaf "true love overcomes all fear". If what you have is real, true and is truly what [both] you want then you can trust. And it's not a trust that has to be naive or blind. A guy, or a girl for that matter can cheat whether you live in the same house or a million miles away from each other. If a persons intent on doing it, distance doesn't have to be a factor in the slightest.
But there isn't a magic point or key that will get you to that level of trust. Nothing magically occurred between me and Joe that got us to that level, it just came from us and what we have. I know and believe what we have is so magic that neither of us would ruin it. That trust has to come from inside you. Maybe to some extent it has to be gained too from both sides of the couple; but that, as in any other relationship comes with time.
Personally I think if you've found a guy [or girl again - just to say i'm not being sexist] that's more then certainly keen and bothered about being in a long distance relationship why would he then go and cheat? It wouldn't be worth their time or effort to keep up that presence.
But then again, what do I know? According to some, just a naive knowledge of love.
image taken from here