What episode does frank find out ian is gay

Frank found out when he walked in on ian and mickey in s2. And yes, he was pressuring mickey to come out but by this point it's shown everyone in Ian's life knows he's gay and accepts it, so Ian only has to hide that he's in a relationship with Mick, not his sexuality.

Ian's journey to self-acceptance and owning his identity as a gay man begins in the pilot episode of "Shameless" when he finally talks about it to his brother Lip.