Just my two cents:
Taeju's rut was triggered by his emotional distress on fear of loosing MC (sorry for the life of my goldfish brain I can't remember his name) His rationality right now is if he impregnated MC, then he would have a reason to stay more than money, that they would become real lovers.
ML on the other hand knows that Taeju is trying to impregnate him but see it as Taeju just doing what ever he wants because he owns him. Taeju already made it clear to MC that he doesn't want children, so what is he trying to do now? Is it just his rut? Or he just doesn't care because he can order to MC to get rid of it anyway? I think MCs fear of Taeju's irrationality and the consequences of it is overcoming his instincts and stopping himself into falling into heat.