As long as both people are comfortable with it there should be no problem.
Having said that the real issue often occurs after one or both of them become involved in romantic relationships. Not many men I know of would want their girlfriend, significant other, or wife to be calling another man (baby, sweetheart, or boo).
I cannot think of any woman who would want her man calling another woman that either. I personally think of "platonic friends" as being the equivalent of "siblings". The vast majority of brothers and sisters would not call each other (baby, sweetheart, or boo). The very use of those of words indicate there is a "mindset" towards that person that is different from that which one has for a sibling.