Most human actions have benefits on one side and cost and associated risks on the other side. Same with marriage.
 There may be plenty of reasons to marry a person you suspect of cheating.
 Like attraction.
 Like companionship vs. staying alone.
 Like getting good genes for your kids - intelligence, health and - yes - attractiveness to the opposite sex (how else would you be successfull cheater?).
 Not that my wife cheats, but there is very little I could not forgive her for the baby she gave me. If she gives me couple more like that I would consider it a good deal no matter what she did - even kill me.
 miko