Question: why is it men tend to cheat on thier gf/wives when they are pregnant? 

Answer: This answer is not intended to justify it and only to explain a possible reason. I think it comes down to natural animal instinct. Having made his wife/girlfriend pregnant his his job is done and his instincts tell him to move on to the next one. The big mystery is why some men give into this urge while others do not.
This question from the Yahoo Q&A forum is categorized in the topic: Health