"Amiga" implies that it's a girl. In English, if we wanted to tease Rafael, we could say, "He has a English girlfriend". Do you think that should be an acceptable translation?
Clearly not. girlfriend = novia. Yes, amiga is feminine, but it's pretty clear that they don't have a romantic relationship. However, the teasing part is completely possible in Spanish too. When a guy says he has una amiga, we often ask ¿amiga o novia? = female friend or girlfriend?