When Josh and I first started dating, he was best friends with a girl named Danielle whom he had never met in person, just online and on his phone. I was alright with him being friends with her... Until she broke up with her boyfriend and her texts to him started to get a little too flirty. I had an acquaintance relationship established with her and I asked her why she was being so flirty with Josh. She straight up told me that she was in love with him. I told her to stop talking to him, and she said "I have no obligation to you or to your relationship. I can do whatever I please." Josh said he had no feelings for her, but I told him right then and there, that he would stop talking to her or I would leave. It was getting in between us and as we were in a serious relationship, we both felt that we need to respect each other and make compromises in order to avoid hurting the other person. So he stopped talking to her... Or so I thought. He has not spoken to her for a year now (that I know of) but after I told him to stop speaking to her, I caught him talking to her on three different occasions over a period of a year and a half. Why fight so hard to speak to someone against the will of someone you love if the feelings are truly only platonic? Wouldn't the person you love, and their feelings, come first in that situation? Likewise, I had a male friend that was always very forward with me when Josh and I started dating, and he asked me to stop speaking to him, and I did. I felt my relationship was more important than allowing a lesser relationship to cause problems. To me, it's all about compromise and respecting the wishes of one another, even if you don't always agree with their wish, you do it to keep them happy because their happiness is important to you.
On the question, though, I do think men and women can just be friends... But I don't think those sorts of relationships are common.