Agamemnon is a Greek king and Menelaos' brother. His unfaithful wife Klytaimestra (Clytemnestra) teamed up with her lover Aigisthos to kill him when he came back from fighting the Trojan War. In the Underworld, he tells us why we should really try to marry a woman who isn't going to betray and murder us:
So there is nothing more deadly or more vile than a woman who stores her mind with acts that are of such sort, as this one did when she thought of this act of dishonor, and plotted the murder of her lawful husband. See, I had been thinking that I would be welcome to my children and thralls of my household when I came home, but she with thoughts surpassingly grisly splashed the shame on herself and the rest of her sex, on women still to come, even on the one whose acts are virtuous. (11.421-434)
Got that? One woman's misdeeds make all the rest of us look bad. (Of course, Aigisthos' treachery doesn't make all men look bad. That would just be silly.)
For more on this guy, check out Shmoop's guide to Agamemnon.