If having epsilon much less sex would make us less happy, then having epsilon much more sex would make us happier, unless we are at exactly the local optimum, which sounds unlikely a priori.

Asr is right, but it actually goes so much further than this.... You have also entirely failed to account for opportunity cost. You aren't just adding some epsilon of sex, you're subtracting from some other area of life to have more time for sex.

And these are just the abstract, theoretical problems with your suggestion. The real-world practical problems of adding more sex are enormous... There are serious bottlenecks to sex. Both partners have to be in the mood or the act has potentially serious negative utility. Synchronizing desire takes a lot of time and effort. (It takes much more than a minute of romance time to yield a minute of sex.)