I’ve never decided whether a guy could be my boyfriend or not in 30 seconds. The thing that gets decided is whether I would potentially, under the right circumstances, ever sleep with said guy. This doesn’t mean I would sleep with him on the spot; rather that I could imagine myself possibly sleeping with him in the future. And there are really no tangible factors that come into play when I decide this. Either I have a spark with that person or I don’t.
As a woman:
It works both ways, you know? 
I think part of it is chemistry, attraction, at least in some way. After a night out partying and meeting new people, I know which ones I want to get to know naked and which ones I want to know as friends. And those that are in the friends category, while that may or may not change with time, remain there for long. Flings may come and go, but friends usually stay around. Which is why I don’t think it is a booby prize either, just different.
I’m a guy, and my reaction is the same.
“Spark” comes first and is pretty well instant. Whether I want to get to know someone, or consider them GF material, takes longer.
I’m not heterosexual, but I am curious as to why you think there are special reasons why a woman wouldn’t date men she’s friends with. They’re usually going to be the same reasons why she wouldn’t date a man she wasn’t already friends with – not attracted to him, personalities don’t mesh well, incompatible personal habits or social/religious/political views, etc.
The only big difference I see is that when it comes to the possibility of a bad break-up, the stakes are higher. Losing a friend AND a lover is worse than losing a lover. If the two have many mutual friends then it could be a similar situation to breaking up with someone who works in the same office: difficult to avoid the ex, lots of gossip from the people around you, and some mutual friends would side with one person and cut the other one off. I’ve been in that situation once before, and I’d be reluctant to risk a repeat.
I grew up with 3 older brothers - I have always gotten along better with guys and found them easier to be friends with.
I always assume everyone is a friend until proven otherwise.
There are times I had let my horniness or loneliness override my common sense and allowed someone who was much better off as “just a friend” or better yet, “just an acquaintance,” wear me down and attempt more than that. I knew I needed more but thought I could “make it work.” :smack::smack::smack: One is my ex-husband, the other became such a toxic friendship after we broke up that I recently had to cut him completely out of my life.
Awhile back, after another failed relationship, I developed a “Relationship Credo”- a way to remind myself what things I found important. The list isn’t ironclad, but it helps to have down on paper.
Two of my closest guy friends now are guys I met through a social network (OkCupid and eHarmony) and were actually instrumental in helping me encapsulate parts of my credo. Because they were great friends - but as boyfriends I would eventually have to fire them, or kill them.
The 30 second “phenomenon” is well entrenched in the eHarmony/online dating world where it is important to have coffee or go for a walk together as soon as possible. Any energy expended on developing a relationship without checking the chemistry element is energy wasted.
It’s a very reliable indicator. Get into physical proximity and let the chemical and neurological inevitablilities do their work. It’s really pretty simple, once you quit troubling over every “nice” person and put your focus on who you respond to naturally.
Understood - so once I admitted what I wanted instead of “giving everyone a chance,” I was lucky enough to find my Dewey and have been enjoying chemistry with the best of them 