Re: one more thing I do not understand
My theory on this is that the AP is more of an escape - you can be someone you aren't normally with the AP because there are no strings like kids, responsibilities, etc. There is less emotional baggage to carry, so it's like, "f*ck it", let's do this!
With a spouse, doing something out of the comfort zone may open things they are afraid to open, because they know they will have to deal with it "forever". Also, they may feel more embarrassed. With an AP, if you get really embarrassed, it's easier to just blow off the person going forward.