This topic came up a long time ago with one of my cousins. She was talking about how later on (during/after college) her guy best friends during high school had come up to her, told her that they liked her, and would like to date her now. She responded by saying that she did like them during high school, but now the feelings are no longer the same. Then she said to me, “I learned that guys and girls can’t be best friends without one or both eventually falling for each other.”
I would say the majority of opposite gender best friends do end up liking the other person or at least imagine the person as their significant other at some point. However, I think it is possible to have a close platonic relationship with the opposite sex. I feel that this generally happens after someone gets turned down, goes through a period of awkwardness, and then the friendship resumes again. Maybe there are still feelings but no longer as intense since they got shut downed. Also, I think this can happen when they have been friends for a very long time. After a while, they start viewing each other as siblings rather than romantic interests.