Seems like you've forgotten what you were talking about. Adultery within the marriage is not the same as an abortion. However, it is a personal matter and the government should stay out of it. Most of the times, it's a He Said, She Said situation, so how is the government expected to know who to side with?
Really, it's not worth the effort.
That's why you get hard evidence, just like in any court of law. Although plenty of effort I think a scorned partner is willing to go through getting that evidence if need be.