I am not a person who is going to say its a red flag because I am not paranoid about being cheated on but get annoyed when people dismiss the ideas that you could date or find a friend attractive. The people you are friends with likely have a lot of similarities to you so if you are both single why wouldn't you consider dating? They likely don't hook up if there is not chemistry or an attraction but if people hooking up with friends was not real there would not be any storylines in the tv show Friends. Like I said if I am in a relationship I am not worried about this unless the person has a history of cheating(which is its own issue) but platonic relationships at the moment might be someone you date or marry down the road.