I did a lot of backpacking in hostels during the late 80's and 90's as a single woman and thought nothing of it. There was one time when I was on a budget camping safari in Kenya and it had been raining a lot. There were about 12 of us and by the 2nd day we were miserable, even though the camps were pre-set up by staff. We stopped at a safari lodge for a quick look and I asked how much a room was, it was expensive. There were 3 Argentinians on the safari who were also asking so I asked if one of them wanted to share a room. We had been on safari so I knew they were OK, it wasn't like approaching a stranger at an airport though. I would be most worried about my stuff and my "roommate" taking stuff from the mini-bar and me getting charged for it.

Random stranger at the airport.... Unlikely.

I have shared rooms on adventure tours numerous times. These people are strangers when first met. Even shared a tent with a 'stranger' who was on the same trip when I did my 8 week Africa camping trip.

Hostels are another example, although people's perceptions of hostels that are more than 10 years old should are irrelevant. Hostels nowadays especially in popular areas have private rooms, aircon, wifi, breakfast, and if dorming, safes and security boxes

Abandoned as a kid
 

When I was 9, I was travelling as an unaccompanied minor on Northwest when the connecting flight was cancelled. Another passenger offered to let me stay with her but, even at that age, was cautious about molestation, though I didn't know that exact word.

I lined up with everyone else and the airline gave me a hotel voucher. I thought it was cool but, in retrospect, they really made an assumption that I could take care of myself. At 9 years old, I was clearly short and did not look like a 16 year old.

I wasn't that smart as they gave me 2 meal vouchers and I didn't know how to use it so I survived on water and found out how to use it the next day for breakfast.

For several years I was an active member of couchsurfing. I stayed with strangers all over the world, and had strangers come over to my place. These were hands down some of the best traveling experiences I have ever had.

Many of the conferences I regularly attend will match you up with roommates (or provide a forum to find your own). I actually like this better than staying with a colleague I know, because people you don't know well are more likely to be respectful of your possessions and lifestyle, and besides, the whole point of a conference is to meet new people.

Now that's interesting. A person picked up in a bar, is safe to some, but not a stranger who is in the same boat as you are while travelling.
Could it be that we hear too many crime stories on the news?

Or is it that as a "human animal" we are more willing to take greater risks to satisfy our sexual urges/procreate than we are to get "routine" (IE nightly) sleep?

Yes but they make you pay a single surcharge and I'd rather not do that. Most cruises will do that or you just have to pay the price for 2 unless the cruise isn't fully sold out. Even in that case, I'm not sure if they give you your own cabin without making you pay extra. Some of them have a matching program for those traveling as singles but still sucks if you don't want to pay more.
I know very few cruises that offer single cabins. There was one I found but they were offering them as small interior staterooms without balcony and I wasn't interested in the itinerary.

I would not generally share a room with a stranger, certainly not by choice - but if, say during a flight delay, it was a choice of that or nothing, I might agree to do it. But I would be concerned for security, both of myself and of my possessions.

Of course, in youth hostels and the like one shared rooms without really thinking much about it. And also, as people have pointed out, one's critical faculties are less engaged during a one-night stand, although for preference it'd be in her place/her room so one can leave when one chooses to.

For guys I'm sure they will be equally okay with bunking with a cute traveler girl too.

Totally agree.... I do not even like to stay as a guest in the home of my friends.... just something about privacy.

As the hostels point was already mentioned, ...don't need to do that again.

Weather delays in Bali and Lanzhou (China) caused me to share a room with a stranger. They both let me open the windows, and also didn't smoke, so that was a bit of a win-win.

Does sharing a bed with your ex-girlfriend count?

I have many times at hostels for backpacking through Europe like most of the people in this thread. But then again I packed differently knowing that I would be staying with others and not to bring anything I wouldn't miss.

Oh yes, packing has got to be different if you are going to be staying in a hostel.
Sharing a room with a stranger or an estranged person is, well, strange.
It is giving up one's privacy and sharing very personal space--not just the room one sleeps in, but also the bathroom, not simultaneously though.

Ditto.

If I want to sit around in my skivvies, watch Jerry Springer while drinking cheap beer and eating Domino's pizza at 2 a.m. I want to be able to do so.

Granted I don't watch Springer, drink cheap beer or eat Domino's. But if the fancy strikes me.....:p

I did it once on a tour to Alaska a few years ago to avoid the single supplement charge as well.

And I've been in train compartments in Russia that sleep 4, and they fill them up. They don't segregate genders, either.