He fed Jace's mother Angel's blood while she was pregnant with him, which made Jace special (much as I hate to admit it). At the same time, he was raising your brother, Jonathan. While your mother was pregnant with him, Valentine fed her demon's blood. And so, it was as if he'd fathered two sons: one light and one dark.
Your love life has very little to do with the grand scheme of things, Clary.