PLEASE HELP; My Wife-to-be Says Her Ex boyfriend Is Stronger In Bed Than Me

Guys, I was just having a conversation with my girlfriend/ wife to be about
likes and likes-not in bed, and while listing her likes, she went on a
story telling episode about her previous guy. The part of the story
that caught my attention was when she said that her ex will do her
for hours without resting. She said there was a time she visited the
guy and he effd her from 12 noon to 5pm, that she could barely
walk that day and was feeling so satisfied and fulfilled. She then
said with all the fuss i was making about effing her out of her
senses, that i came short of satisfaction, and that the guy was
stronger in bed. You could imaging how furious and boiling i was
feeling inside while hearing this from her in a normal conversation.
The facts are though that when ever we have sex, i am absolutely
sure i last for atleast 15 minutes or more no doubt, my joystick is
5.7 to 6 inches erect and hard as wood, i do pre-intimacy and such,
so what the hell is the problem? Is it even my damn problem or
her's? I know for damn sure no normal guy will last for more than
20 minutes of constant thrusting. If she has to be effd for hours in
order to feel satisfied, isn't she the one with the problem? On the
one hand, i feel like i shouldn't even worry myself since she is
aperantly the nimphomeniac not me. On the other hand, i feel that
her "accidental revelation" is a sign of what's to come, as in she
might be a cheater when married because of her insatiable appetied
for marathon sex.
what's your opinion guys