Willow made an emotional commitment to a "person" sight unseen. Yes, that's a dice roll, but she was all-in, and it's not Moloch's fault that he is a charismatic Toaster thirst trap.
Well, maybe it is, but what choice did he have?
Helo from BSG Knocked up a Toaster, and he stuck it out, because he is a man of convictions that follows his dick into whatever calamity it generates. He's a good boy.
True, Sharon is prettier than Seven of Nine, but it is unlikely that that reject go-bot Moloch was in was Malcolm's final body. The technology was out there. In a year Malcolm could have looked like young Colin Hanks, and in 4 years Mally could have looked like young Christina Aguilar, when Willow came into her own.
They might have had a successful physical future together, if she had just waited for the parts to become available, and he didn't sacrifice her to evil.