Sex in my opinion, is far more mental than it is physical. You may be incredibly attracted to someone physically, and the sensation of your bodies may be incredible when you are together, however if there is something going on mentally, than it can outweigh all of this.
For instance if I am incredibly excited about something mentally and the right thoughts are coming through my mind I can cum in under a minute. If I am super horny though and incredibly into something physically but my mind is not completely in it (depression, thoughts elsewhere, worries, stresses, anything that can deplete your mental resources) then it doesn't even matter, I cannot get off unless I strain for it and then it's just not fun...