If friends are comfortable enough with each other to get to that point, and they aren't already in a relationship, then they should be able to explore those feelings with each other. But, some form of feelings will develop, it's unavoidable when you're that intimate with someone. Those friends may be closer in their friendship, or maybe other feelings develop. It depends on the people involved and their feelings going into it. But yes, friends should be able to have sex with each other only if they're ready and if they're willing to see what kind of feelings, if any, develop after.