Yes, BUT...
In a long-term friendship between a healthy, straight, reasonably okay-looking pair of a guy and girl, eventually there's going to be some sort of sexual tension, especially in younger (late teens to late twenties) years.
Why? Well, why wouldn't there be? Chances are, they'll be in and out of romantic relationships, they'll experiment more or less when it comes to... lots of things, including sex. And sooner or later, they'll each start wondering: I really like this person and we get along so well--why not try it with them?
Or maybe it's just been that way with every single long-term guy/girl friendship that I've been in, or been aware of.
Of course, I'm sure there are exceptions to it, especially if they know that the other is "off-limits" (already have significant others). But like I say, people tend to be in and out of relationships enough, so if the timing is right... look out! :)
