Isabella conceived him after she ran away. How many times does Nellie observe that the blonde Linton looks nothing like Heathcliff? Heathcliff was content to take advantage of Isabella's infatuation with him to further his revenge schemes against Hindley and Edgar, but he also made it very clear to Nellie that he completely despised her, so in spite of the possible Fridge Horror, it doesn't seem likely he would ever have wanted to invoke the Marital Rape License against her. Of course, Heathcliff wouldn't tell anyone this because he needed an heir to complete his schemes of defrauding Edgar and Cathy (II) of their land.