This a good question. I would say that there are two points that are worth considering. 

First, Agamemnon is coming home after the long Trojan war. However, the first death that he encountered in the war was the sacrifice of his daughter (Iphigenia) to obtain favorable winds to sail to Troy. So, when Clytemnestra speaks of their son, Orestes, his daughter is also in the background. So, this quote is uneasy to say the least and it forebodes danger. 

Second, Clytemnestra is already plotting the death of Agamemnon. So, these words might sound fine  - especially the words "Our true love's pledge", but nothing is further from the truth. Soon, Agamemnon will be murdered in a bath. 

In short, the quote is setting up the lamb to be slaughtered.