Well first of all, I strongly believe that men can only be friends with females "off limits" by the man's personal code and females they want to "have a chance at". (The first group sometimes overlaps with the second of course). So the mere fact you're "midly attractive" and roam around "man events" thus meet a lot of men who feel like they've got "home court advantage" will make a lot of them approach you or try to "have a chance at" after only knowing you for a short time as "friends".
A men's personal code can be wifes, girlfriends, a number of dates/time passed, because you remind him of his sister/grandma/whatever or even, like me I suppose, because they know they may not be able to make you truly happy in the end. A men's code can of course also barely have anything like what I just said (which sees to be a growing trend among men sadly) and that leaves just the woman they want to make whoopie with.
I recognize the being interested in someone and "otherwise I'll just stay single", because I think if at this stage in life/knowing the people I know I got into a relationship and even married/children with someone else and this person 20 years down the line (not drastically changing of course) would come into my life and I felt I "had a chance at it" I'd take it.
Someone "better" can come along of course, but they would only take that first person's place if they remained unattainable too. I rather be single than in something I can't plan "till death do us part" pretty much.
But I think all men also have a level of this. I'm pretty sure that at "any event" your average male will scan the room for "his pick" and if that lady ends up being a no go, they'll close shop for the night at that place. Even if the "second pick" is still "better" than the "first pick" at another place. I suppose the term at which men look at their relationships/selection process is what only really differs in men on that level.
So in (my) conclusion; if you're a female with a guy friend, he 99% sure wants to make whoopie with you, and the only reason he hasn't attempted yet is because he's bound himself by some rule, or just because he hasn't found the moment yet. You "can" be "friends" with the rulebound guy I suppose, but if it happens to be a rule with some kind of expiration possibility that holds him back, he'll go for it as soon as it's lifted, (or booze). (or at least move to the "hasn't found the right moment yet group"). Finally, I think all people, or at least men that I know off, have the "if I can't have my number one pick, then I rather have nothing", but for how long and from which group/ect they make this pick is different for everyone.
Let me know if this helped.
Edit: ugg, spend too many hours and breakfast typing my reply (no one had yet when I opened this) that by the time I posted it, other people already pretty much gave the same advice and said the same thing in half the words
edit edit: so I removed my advice on making yourself look less attractive to not be "a pick" or completely offlimits by having for example a guy friend with you.. well now I said it again but in less words.
edit edit edit: just because I can!