This turns out to be a huge powerplay for the guy. He indirectly states something that could be interpreted as "he doesn't care about the relationship" and this leaves the girl thinking that his commitment can end at any time.
Guy likely thinks his girl is 'crazy like all girls' and was being all emotional over nothing, and unable to have a logical discussion. To him it is obvious that he cares about her and is enjoying the relationship - they just had sex all day. But he just doesn't understand what went wrong or how to resolve it.
Now, lets be honest here, after 2 years of a beautiful, fully committed relationship this made me almost burst out in tears.
I had no idea where the FUCK those comments of his were coming from because I had never cheated on him, there was never any inclination that I was, he has never asked me if I am and I would never do such a thing. The concept of cheating is soooo far from the realm of our relationship that its not even a thought.
He believes what he said (or is telling himself what he wants/should believe out loud); that he accepts that he cant control her 100% of the time, and therefore wont let her actions have too much power over him. I believe that he tells her as he is just sharing his worldview (oxytocin from sex + I've made similar mistakes myself) and trying to rationally explain to her how she should see the world in order to avoid getting very hurt in the event of breakups. He likely cares about her and is opening up.
However, he indirectly communicates that she might be in other relationships than him in the future, therefore saying that the current relationship might not last forever. This is all the girl hears as they constantly picking up signs of commitment or lack thereof. Men tend to be blind and just notice sex stopping.
The conversation now splits into two. The girl will talk only about the subtext, but will not be able to explicitly state what the subtext is as it puts her in a very powerless position to say "this is a discussion about you stating you are not committing to me or would not be hurt if we broke up". Her immediate thought was that he is cheating or thinks that she is cheating - "why else would he not care if I cheated than if he had cheated himself? or does he think i cheated and is trying to trick me into saying that i did? or is he trying to be above me by saying that he wouldn't even care if i cheated?".
It is up to the guy to figure out the subtext, but he is too busy in "logical debate" mode. He likely will see the discussion being about "whether you should fall really hard for somebody or not" - in which case he believes is best for people from his experience (which she lacks). He is debating/arguing his case and thinks she thinks that she knows better, so must explain that she has less experience and should agree with him and change her views.
He will argue his case and be annoyed that his girlfriend's responses lack logic. She will be upset at him continually repeating a subtext statement that she finds very upsetting (especially after a great day of sex and tv together - "why is he saying this to me?!"). She will try to place the situation in one of two categories: 1. he is cheating on her/about to leave 2. he thinks she cheated. She will not find an answer that fits in those two buckets and be upset then go to reddit for advice later on.
Guy might equate honesty with trust and generosity. Girl probably will not understand that.
As you would expect the comments in that thread are all: guy is asshole and probably cheating.
Guy understands that as much as the girl is in love with him now, it can disappear fast. Girl does not know / cannot see this.
Often when your girlfriend becomes very emotional for 'no reason' the underlying cause is doubt over commitment. A little doubt is good, should never be 100% comfortable as you get lazy, and she wants/needs to feel some emotion. A lot of doubt thrown in at a happy time for seemingly no reason is bad.
To simplify heavily: Direct/Indirect statements of commitment (or lack thereof) are your currency, and her currency is sex. The exchange rate is a factor of your alpha-ness.
Conclusion: Dude made a mistake talking to her like that. Should not say things that can be interpreted as commitment starting to vanish unless the girl has negative behaviors. Hurt her feelings at a time he should have offered positive reinforcement and love. Dude likely will not figure out what the misunderstanding was, and neither will girl. In order to resolve situation one will have to really try to understand the worldview of the other. Until then she will be just another 'crazy, emotional' girl and he will be just an 'insensitive asshole' guy.