He wasn't furious about Mark and Becky - just not happy with the idea. Did he really believe they hadn't at that point? Did he expect Darlene to still be a virgin at age 18?

I think he was mostly about being lied to by them both. Darlene was his daughter and he cared a lot about David.

Dan's intense anger toward David and Darlene in college was fueled less by the act itself and more by a sense of betrayal and a fear that Darlene was repeating the family’s cycle of struggle.

Key reasons for his reaction include:

Prolonged Deception: Dan was furious because David and Darlene had lied for months about living together in Chicago. He believed he was closer to Darlene than his other children and felt a deep personal betrayal when he realized she had been deceiving him with David's help.

Fear of "History Repeating": Having already "lost" Becky to an early marriage and a stalled future with Mark, Dan was terrified that Darlene—who had earned a scholarship to an elite writing school—was throwing away her potential for a boy.

Protection of the "Favorite": Darlene was widely regarded as Dan’s favorite child, and he was historically more protective of her. While he eventually accepted Becky’s situation as a fait accompli, he held Darlene to a higher standard and "expected more from her".

Lack of Masculine Respect: Dan often struggled with David's sensitive nature and "less masculine" interests. Finding out that David was living secretly in Darlene's apartment made Dan feel that David was not only being sneaky but also being "dominant" or manipulated by Darlene in a way that Dan found disrespectful to his household rules.

Regarding her virginity, Dan likely suspected the truth but chose to maintain a "willful ignorance". When David moved into the Conner house, Dan explicitly stipulated there would be "no smoochie-woochie," showing he was aware of the possibility but refused to acknowledge it until the lie about Chicago forced him to face it.