Do you remember which version you played? The dev seems to have edited the code for version 17.2:
I have the latest version 17.3, I played around with Clara's lust level and if her lust was high (>=9) then the game just skips to Clara and MC starting to have sex after Frankie calls for Clara's attention and says she has an idea. However, if Clara's lust is low then the game skips the part where Clara considers whether to hold Joe's junk (lust >=11) or stay faithful to you (lust <11). It just skips to the choice menu where she's already holding Joe's and you can choose whether to stop Clara or let her continue and have some fun with Frankie yourself.
The correct order of the couch scene during Joe's dinner should be like this:
1. Frankie calls for Clara's attention and proposes that Clara hold Joe's junk.
1.1. If Clara's lust is lower than 11 then she refuses to hold Joe's and you two can have normal sex.
1.2. If Clara's lust is at least 11 then she accepts and proudly holds both cocks.
1.2.1. Choose to stop this and just have sex with Clara yourself; -1 ntrQuest point
1.2.2. Choose to let it happen and Clara either performs only handjob on Joe (lust <13) or blowjob (lust >=13). You can have a go with Joe's wife and later have sex with Clara. +1 ntrQuest point I'd argue that partner swapping equals cuckolding though