It isn't strange at all.
Time and time again, how many times has it been proven that a couple can fuck 10x a day and it doesn't mean it will generate any discussion or interest if there isn't anything interesting going on.
You like to bring up Katie as an example: she was hot with a hot body and had a lot of (boring) sex with her boyfriend; in the end it meant nothing. Nina and Alan had sex multiple times a day when they first moved in and still ended up in the bottom or near the the bottom of viewership. Look at Ben and Whitney; they fuck multiple times a day nowadays; nobody cares anymore. Same thing for Clair and Patrik.
That shit ALWAYS gets boring after a while. Maybe not for everybody, but enough people will tune out once that shit becomes a routine, and and Otto and Lola apartment is super routine. They hardly invite guests, they don't throw parties, they don't do group sex like they used to. So why is it strange that the topic died as you say?