logic dictates that if you call united to book a ticket then there’s a high probability that you want to fly on United. Someone over the phone can’t read your mind and assume that you are calling United to book a flight on TAP. The agent went through the most logical thought process for the situation and presented a better option that the majority would consider reasonable. There is no excuse for the OP to treat the agent rudely, especially when he’s literally the only person in the world who will ever make that request, and they can’t understand why.