Rejecting an intimate suggestion appropriately is difficult because it is natural for a partner to want to escalate the intimacy from one level to the next. Moreover the communication is often through body language rather than the spoken word. What complicates things is that she can't be expected to know what you are thinking:
I wasn't ready at the time (...) I said "I don't know", which made her respond by taking my hand and placing it (...) The reason for declining is that I don't feel comfortable yet to do the action (...) Slowing down is important, so there is no feeling of having it rushed. Enjoying each others company is central to this.
With due respect to both parties, it is clear from your narrative that your GF of that incident decided to ignore your brief and inconclusive "I don't know" response (which actually expresses none of your justified sentiments, and without which explanation, "I don't know" can easily be misinterpreted as simply your lack of initial confidence) and went right ahead to implement her 'intimate suggestion.'
That was a positive action on her part in the context of your mutually acknowledged intimacy, and only serves to emphasise her original invitation, but you would need to decide how to react to her bold and decisive move. These are the 4 options I can think of, which seem to cover the range of possible reactions to this situation:
- Option A (not recommended because of your justified reasons for declining, as articulated in the question): you can go along and do what she wants. Your hesitancy might be apparent in your body language. Doing something you are not convinced about, just to please another person, is not usually recommended but it might possibly be good in such situations if you were simply lacking the confidence to make your move. The result can be known only by carrying out the experiment.
[Statutory warning: If it is a socially/ legally/ ethically controversial situation, you shall have to take the sole or joint responsibility (depending on the circumstances) for following through on your partner's intimate suggestion, and shall be considered liable for the consequences of that decision.]
- Option B: you firmly but gently take back your hand and say, "sorry but I am not ready for that yet; I don't feel comfortable yet to do the action. Slowing down is important, so there is no feeling of having it rushed: enjoying each others company is central to this," which is basically your explanation in the question here.
This approach was recommended by our highly respected member @apaul in his earlier answer. What is good about this approach is that you clearly express your sentiments in a way that is explicit and difficult to misinterpret. What is possibly not good is that the emotional momentum can get broken and GF may not like it one bit! You are basically taking a certain amount of risk by verbalizing in an intimate situation, when not speech but body language predominates, and emotions can run high. However, if your partner understands and appreciates your position then it is an excellent result.
- Option C: you firmly but gently take back your hand and say nothing. Your body language expresses your sentiment. This is not very likely to be misunderstood in an intimate situation. Ideally your partner understands you and is not discouraged by your decision, but if the other person persists in getting 'more intimate' then you may need to physically disengage (firmly but gently) to demonstrate your position. You can express yourself verbally and fully communicate your sentiments later.
I see this is the sort of thing you would prefer yourself, based on your observation in the question:
I would have preferred a simple rejection of the proposal and get back to it later.
To avoid any misunderstanding, and in the interests of keeping the relationship healthy, you should clearly communicate your reasons to your partner soon after the incident, and emphasise that despite your preference to "take it slowly" you really appreciate and care for the person, and value the relationship. You would benefit from learning to discuss such sensitive topics with your partner as your interactions progress, as pointed out by @ShadSterling in comments.
Note: whether you choose option B or option C, which are the "recommended" options, you should be prepared for your position to not be appreciated, and for the relationship to cool down quickly as a result, as someone noted in an earlier answer or comment. But if the other person understands, appreciates and agrees with your point of view, it can really strengthen your relationship.
- Option D: melodrama. Especially a girl from certain Asian cultures might shout and scream that she is being taken advantage of against her will in an intimate situation, though men in your position can make the same accusation.
But creating a scene is bad IPS and never recommended on Interpersonal Skills Stack Exchange.