I feel like the thread's title and the OP ask entirely different questions.
"Can men and women really be 'just friends'?"
vs
"Is there always be a risk/possibility of being sexually attracted to someone of the sex you are attracted to?"
I'm going to answer the second question, because I feel like a lot of other people have answered the title and have mirrored my feelings that it's a bit silly to question whether it's even possible for a man and a woman to be friends.
We know that homosexuality isn't a choice, right? Can we reasonably say then that heterosexuality and asexuality aren't choices either? I feel like that's a reasonable assumption to make. We Also know that men can't control when they're, ahem, aroused.
Taking what we know and applying it to the situation, I think it would be fair to assume that people generally don't choose who and what they're attracted to in general. Therefore, since it is not within our control, the answer would be yes; we are always 'at risk' of developing feelings towards others, especially of the sex we're already attracted to. One's tastes can change over time. Not even sexuality is absolutely set in stone.
Those are my thoughts.
Does one really treat one's female and male friends the same way? Related thought.
I think there are two ways to look at it.
You can take humanity out-of-context and say that yeah, we kinda treat each other more or less the same way in the grand scheme of things.
Or you can look at it through the context of the males and females themselves and say that no, we actually kinda don't when you think about it. I prefer the latter viewpoint, because I feel like the context is very important for the purposes of obtaining a relevant answer.
A person
could treat men and women the same, but in practice I feel like that's kinda hard to do without putting a lot of effort in. At the end of the day hormones are very powerful things and the existence of heterosexuality and homosexuality are pretty good examples of people not treating men and women the same. Our hormones have a fairly strong effect on the way we think and feel, which is incidentally why I find the pill terrifying, so there's a real chemical difference up in our heads. Men and women, much like different races, also have very different histories that shouldn't be forgotten.
So no, I don't feel like we treat men and women the same. Even when they're our friends, there are differences that are only subtle when you remove context. There are a lot of situations in which we do and a lot of situations in which we should, but there are also a lot of situations in which we don't. In my honest opinion that's perfectly fine. Equality is a great ideal, but it's not a reality and in some cases treating everyone the same would mean ignoring reality.