Marriage: One unfaithfulness exception, or none?

[garcia1000]

Hey guys.

In the coming few years, I may marry with my girlfriend. She is really nice. One thing I am thinking of is, say, after we are married, one of us has an affair. At this point, it looks extremely unlikely, but after reading some threads elsewhere I thought that I should better plan out my flowchart.

How damaging is it for someone to have an affair? Does this change if there are children? Is it automatically divorce if that happens? What would be any mitigating circumstances?

I am thinking of saying that we each get one chance, and after that it's instant split. Is this good? Will the marriage survive if an affair is had and discovered? Would it be better to have no chances instead?

I know that wanting to have an affair is a very human thing, and is natural. We have to resist the temptation, but it can be hard for some people. Any advice?

I am sure that I can bring this up with her without causing problems, our relationship is good.