"저는 앉는 남자하고 이야기하고 있어요."
Translation:I am talking to the seated man.
When ~하고 is added to the end of a noun, it means "and" or "with." Without it, it would be translated as "I the sitting man is talking," which wouldn't make a lot of sense.
They use the word "to" because it is more natural in English to say it that way, but I think it would be better to translate it as "I'm talking with the man who is sitting" or "I'm talking with the sitting man." "Sitting" rather than "seated," especially because they used the present tense 앉는 rather than the past tense 앉은.
am i the only one that thinks the -하고 is there because it means you're having a conversation because 이야기 means a conversation or a talk so that you're in the process of having a conversation with a seated man?
I don't think in this instance it is saying "with" even though that is implied since you need two people to have a conversation
just my opinion.