I have had asthma my entire life, and have been married for 40 years. If you and your future husband love each other, asthma will not cause a problem for you. I agree that he ought to work on his asthma control, but if he's a doctor, I expect he is fully aware of the importance of medical treatment.
There are many things that will create problems and stresses for your marriage, and asthma may be one of them. Marriage takes work and commitment. If you're ready to work a your relationship, and have a loving, committed relationship with your future husband, you'll do just fine.