I think they meant like, since he already had a lover when the fl said she wanted to marry him, he could've just declined her marriage proposal and propose to his lover
(though since she was the duke's daughter and he was of lower status than her, her father might've demanded or threathened him for the marriage - but then again, if that were the case the duke could've also threathened him to stop seeing the other woman, which makes me wonder the circumstances of their marriage - well, gonna find out hopefully in next chapters)
Either way, yeah, the fl was kind of delulu and blind in this one, kind of dumb of her to believe that his "love" is temporary and only hers is permanent, and surely he couldn't love another woman - a fact she realizes too late that she was narcissistic/egoistic for thinking the world revolved around her. I agree with you on that.