Never have I ever seen a 100% plutonic relashionship between a man and a woman. The only time that a guy will ever JUST be friends with a girl (and vise versa) is if that person is a family member (those are usually off limits North of South Carolina), a friend of the person you are involved with or interested in (we always want to get in good with the people our significant others will complain to), or a co-worker (but that's not really a friendship is it? If you take it outside of the office more than is necessary, you probably want more than just help getting the copy machine to work).
The only reasons that two people of the opposite sex would make any kind of investment into a relationship with one another are rooted (at least at first) in a romantic interest except for the exceptions I previously laid out.
To quote one of the greatest explorations of this issue on film: "[N]o man can be friends with a woman that he finds attractive... Yes, that's right, they can't be friends. Unless both of them are involved with other people, then they can... This is an amendment to the earlier rule. If the two people are in relationships, the pressure of possible involvement is lifted... That doesn't work either, because what happens then is, the person you're involved with can't understand why you need to be friends with the person you're just friends with. Like it means something is missing from the relationship and why do you have to go outside to get it? And when you say 'No, no, no, no, it's not true, nothing is missing from the relationship,' the person you're involved with then accuses you of being secretly attracted to the person you're just friends with, which ...you probably are. I mean, come on, who the hell are we kidding, let's face it. Which brings us back to the earlier rule before the amendment, which is men and women can't be friends." - Billy Crystal in Rob Reiner's "When Harry Met Sally"