its your call at the end of the day, yes their are councillars etc to aid/point you in the right direction, but you will ALWAYS have doubts, thats human nature, you said it was a one night stand and he did'nt even no her name, yea right!!
its amazing how people in life are SUDDENLY forgetful when they have done something wrong, it comes down to how forgiven you are, law of averages will say i,m afraid that he will more than likely DO IT AGAIN as hes had a taste of it
It comes down to YOUR TRUST against HIS TEMPTATION,
the latter being the favourite to fail