keyhole trunks

i went to the supermarket with kezia this afternoon as a father/daughter bonding thing. we walked past the men’s pants department and i saw these keyhole trunks.
trunks seems a modern word for pants (and i use this term in the most british of senses – non of your trousers here pls).
and what’s the keyhole bit all about ? is it for those physiologically challenged ? or a modern day chastity belt ? it’s all too much for me to comprehend.
next they’ll be saying men need to wear single piece swimming costumes at public pools.