What happens when you get lucky with 3 relationships in a short span of time rather than having one in your teens, grow, another young adult, grow, another as adult, get married, happy life. No wonder they only ever talk about sex when it's all they relate with a relationship. I genuinely think that this "deeper understanding of a relationship" is really just coping with everyone involved usually being unlovable losers who found too many sexual offers in too short a span.
Ever wondered why they stop at 2-3 extra lovers? They're likely not interesting for most people and got lucky in a short timespan. "Woah everyone loves me! Better slow down haha!". It's coping and settling with having little genuine romantic success.