Why don't people get it!

Alright, so I've been answering some questions on some dating forums for people and I'm amazed at how they don't see what to do while their writing their questions out. For ex., a guy said he was into a coworker who he's pretty sure isn't into him (not to mention their both married and he still loves his wife) and he wants to know if he should continue to pursue her or just remain friends. Well common sense says to not cheat on your wife and leave the coworker alone. I don't understand why some people think it's ok to cross certain boundries when if the tables where turned they'd be freaking out. So my question is at what point do we stop and ask ourselves if we're doing the right thing?