Could be any number of things, distraction, the woman laying there like a dead fish, too much ****, etc.
Not necessarily. Maintain an erection is a fairly complicated physiological process. Things unrelated to his attraction to the woman can cause problems.
No, sometimes it means he's stressed<br />
or distracted, maybe only momentarily<br />
could be a medical problem<br />
Could be he's getting tired, or sore... maybe having TOO MUCH sex in a short time span<br />
or yea - possibly also that maybe he's losing interest.