When the wife and her boyfriend are trying to conceive

I would say its not uncommon for such things to happen. In a long term cuckold relationship, its far higher that the woman gets pregnant with her lover than husband or boyfriend.

In a hotwife situation it could differ.
 
I'm in a situation now where I'm getting ready to impregnate the Mrs. while her hubby takes a back seat. We've been in a full cuck/hotwife relationship for about a year and half and it's only been about a month or so since she's come out and outright stated that she wants a baby and for it to be mine.

For a bull like me, it's the ultimate prize in my opinion and ultimate in cuckoldry. It's animalistic primal, and very hot.