When my wife and I first met, we quickly found out we both had been in the lifestyle before. So that, coupled with my IR preference, and her letting me choose play partners, has led to her being pretty much black only.
She got introduced to swinging by her ex husband. For her it was like opening Pandora's Box, but he quickly found he didn't like it. That, and other relationship issues, led to their demise.
For me, I've always been very sexual, so it was natural for me to migrate into the swinging lifestyle.