Well, in a planet with a reduced living population, you need new rules. I don't see anything wrong with them having a relationship--but they are on different wavelengths.
We're not talking about in-universe rules. We're talking about real-world rules. The network and advertisers aren't going to take "in their world it makes sense!" as an answer if they're uncomfortable with the show having a middle-aged man engaging in a sexual relationship with a teenager.