In human males, there are two basic reproductive strategies. There is the standard pair-bonding method and the "spread your wild oats' method. (The latter method explains why a guy will have causal sex with a girl then call her a slut afterwards. It makes evolutionary sense to breed with her, but not to invest time and resources forming a pair bond, but I digress.)
In a pair-bonding situation, it would make sense to continue to support the woman during pregnancy by definition, otherwise it is not pair-bonding. Putting all your effort and resources into mating with as many fertile woman as possible would be an exclusive "spread your wild oats" strategy.
Also, a woman who is pregnant, or has had children, proves that she is fertile. A woman who has never gotten pregnant might bear more children, but there is a chance that she is sterile, and if you marry her, you might become a genetic dead end.
Also, pregnancy attraction might be related to breast attraction. In almost all other mammals, the female only has enlarged breasts when she is nursing. Therefore, men should be attracted to small breasts, and evolution should cause women to be flat-chested, or rather, remain flat-chested like the apes. The causes of pregnancy attraction probably parallel those of breast attraction.