Yes people can bitch about something if they don't get what they paid for. The problem is people subscribe to RLC expecting something that is NOT guaranteed. You are half correct that these couples sign up to live in these apartments and are getting paid, or getting some sort of compensation, but no where does it say that these couples are paid to have sex. There's no regulation on how a couple should have sex, or where they should have sex, or how frequently they should do it. Look at Masha/Sasha or Nelly/Bogdan that have sex very frequently. Then you have couples like Maya/Stepan or Dasha/Demid that only fuck every so often. These couples are only here to live there lives. Yes sex is included, and to some small degree expected since they are in relationships, but sex is not mandatory. There are 15 other apartments. Each one brings something new and different to the table. If you do not like what you see in one apartment, don't watch it and watch one that interests you.
And at the end of the day, no one is going to be completely satisfied with an apartment. People will literally always find a reason to complain about something.