Marriages got ruined before the internet also, so I highly doubt that “all” ruined marriages are due to the internet. It would also be very interesting to compare the frum divorce rate in 2012 to the frum divorce rate 20 years ago. Basically, I suspect you won’t see much of a difference, and if there is any, I highly doubt the internet has anything to do with it.
Anyway, here’s another reason: I wasn’t always frum, so I’m not too afraid of the big bad world out there. For an 18-year old boy from a frum family who hasn’t seen anything but frum Gateshead, Lakewood, or Boro Park and who has never spoken more than 3 words with a non-Jew and never spoken more than 2 words to any female who is not close family, things might be different.
Yeah, I know people say “that’s the yetzer hora talking”. I’m sorry, but I don’t really care. I know myself, and I (sadly, unfortunately) make my own decisions. And yes, I know I should ask a rov, however, being my Dutch arrogant self, I am my own boss and make my own decisions – I could never accept anything I do not agree with. Sounds bad, maybe it is, but that’s just the way it is.