I wonder if it's also easier for people who have less family relationships going on.
Me and my girlfriends are all distant from our families. If I regularly visited my parents and siblings and spent time with them, I'd have less time for romantic partners.
I wonder if there's a need for secure family-like relationships, and where the family of origin has failed, people are more likely to seek that feeling from romantic partners. "There is no form of incest greater than T4T"
anecdotally, all those I know who practice poly, and name it as such, also say they have asperger's.