This has always been the most difficult thing for me to get my head around when speaking of these types of cases. Historically, these cases have stemmed from a participant saying "no, stop" or something along those lines. More recently, we speak more and more about giving consent in the form of "ok, yes, we can have sex", etc.
I struggle personally analyzing these cases because "giving consent" and "lack of consent" are two different things for me. If I try to apply them to my personal experiences, I am not sure I can recall a time when a partner has said, "we can have sex" or "yes, you can insert your penis in me." Everything was organic and just kind of progressed. On the other side of the coin, I have had instances where things were getting passionate and a partner specifically stated, "hey, I am not ready to have sex" to which you know where you stand.
I think many people have sex with lack of consent and am not sure lack of consent can be communicated. If a participant says no, that is non-consent for me and not lack of consent. I know it may seem like semantics, but I think this is where many of these cases get bogged down.