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