Personally, I almost understand your dilemma. Nevertheless, I do not condone your situation. By that, I mean this: You are married for better or worse. I am not married and almost envy your situation. I would not find my wife any less appealing given the circumstance. Nevertheless, I must admit that given your situation you are being unfair not only to your wife, but in turn yourself.
Let's explore why:
First of all, the reason you got married, what is it? Well, biologically, your primitive subconscious desire to become married is to have sex without having to challenge for it. Consequently, sex=procreation. Now, was your reason to get married to meet the human requirement to procreate, or was it to have some hot chick on lockdown for your own sexual desire? Reason I bring this up is simple. Simply, if you loved her, you would overlook her pregnancy. You made mention that she was slim and therefore I logically deducted that you were attracted to her because of her slender build. Her looking like, as you said, "swallowed a baloon", indicates that you perhaps married for the wrong reason. If it were me, I would still be attracted to her. I would possibly (because I am not sure because I have never been married, nor do I have children at the age of 32...) not be affected at all.. As twisted as it may seem to you, I may still be sexually attracted to her. Why you may ask? Simple, I made it a point to visualize and perhaps precognate the situation given my understanding of the way the female body changes considering pregnancy. If you were to have given yourself the opportunity to test yourself mentally, you could perhaps have prepared yourself for the occasion you are currently presented with. By my discernment, you are not truly in love with your wife, and therefore you have placed an unfair burden upon yourself. My percentage ratio is this: 33.33334% unfair to you and 66.666% unfair to your wife.
The solution to this dilemma is to face what you fear. I am no psycologist, but a drill instructor... All fears I have, I face. Some fears must be faced head-on. How you face your fears head-on, you may ask? Simple... Talk to your physician, see if it is safe to have coitus with your wife given the stage of her pregnancy. I only say this on a logical level... Women tend to have a different hormonal balance while pregnant, and often become a little more sexual in the realm of pregnancy. Given that you may be delightfully suprised.
Finally, I must say this... You are awfully concerned with her losing weight... What, you going to divorce her if she don't do it? The best way for her to lose the weight is that you be a little bit more nurturing while she is in her current state. She may get depressed and depression in women often leads to heightened cortosol levels which are fat retainers. Her stress will keep her fat, so if you are really concerned, make her pregnancy as stress free as possible!