Did your mum take a break from paid work for raising you? I think this is a large (not sole) contributor to the effect (certainly in the past) - that women would often be raising children at home and naturally fall in to socialising amongst each other through their shared situation. Men, then will often focus on work to make up for not being at home, and to ensure a good household income -- which can lead to being too tired to socialise, which quickly leads to losing friends.

Would be interesting to see some stats to see if this narrative hypothesis is born out; do married/monogamously partnered men who become sole breadwinner lose more friends than those in other situations.