I think we can all see that if one or more of the potential sexual partners is going to change their mind, it's always better to do this sooner than later. The sooner in the process, the less awkward and unpleasant the experience is. That's a no brainer.
But there is no 'one-moment' in which a person's autonomy and right to change his/her mind stops being relevant. Not when you've been invited in for coffee. Not when you're naked. And not when you're actually doing the deed. Is it unpleasant, unsatisfying and awkward to stop at such a moment? Undoubtedly, but that's the chance you take when you try to have sex with a human with cognition and a will and choices of his/her own.
Now, it may be easy for me to talk like this; never having been asked to stop in the moment of passion... And I can imagine it not being nice or even easy to come to a complete stop... But once consent, at any time in the process, is out the window; you should stop. If the places were turned, and you change your mind, no matter how unpleasant that may be for your partner, you'd want them to stop as well. Simple as that.
Consent is a fairly simple matter.