I agree with your second sentence; they can become friends if they don't think that they can be romantically involved. That doesn't mean that they can't be courting someone else to be married.
It seems like your first and second sentence contradicts each other. Also in your first sentence it seems like you are saying that they can't be friends if they are romantically involved which is the opposite of what a marriage should be.
Ronald, I said a single man and a woman "can not be just friends as two person of the same gender " because usually, one of them or both develop romantic feelings to each other. Second sentence...many people decide to be just friends when they don't have any chance to be romantically involved with the person they love (Platonic love) and of course they can be courting someone else to be married.