In order to decipher this mystery, you need to ask yourself a lot of questions. The first thing to ask is, did you ever feel passion at earlier points in your life? If so, when.
Next figure out when those feelings began to fade. You also need to find out if you feel sexual passion during intervals when you aren't with him. For example, do you get hot when you masturbate.
The key is to isolate whether the lack of feeling is situational (linked to being with him) or whether it is across the board. If the lack of feeling occurs with him alone, then you will need to examine why your mind is turning off your body.
Sexual numbing is a form of psychological anesthesia that is designed to protect the self from a perceived danger associated with allowing oneself to feel sexual feelings. The danger can take many forms: fear of closeness and intimacy, which itself may conceal a fear of dependency and/or a fear of abandonment. These emotional issues are best resolved in therapy.
Let's say that your loss of desire is across the board. You will need to examine whether your problem is due to emotional causes (stress, depression), various physical conditions (hormonal imbalances associated with peri-menopause, low thyroid function, diabetes, systemic yeast overgrowth caused by pregnancy, birth control pills, antibiotic use or steroid drugs, etc.) or a combination of both.
You may need a holistic doctor to help you sort out if something is going on physically. If you analyze the problem systematically you should be able to get to the bottom of the cause(s) and resolve them. Let me know what you find out.