AICM, would you mind telling us how long you have been a mumsnetter? I mean, have you changed your name for this thread? Because your situation sounds so bizarre that it does come across as though it's completely made up.
I have looked up pituitary glands and it doesn't mention loss of libido although it does affect your hormone levels. If you have such a benign tumour then I am very surprised that your GP is happy to let it lie. Have you been to your GP lately?
You can obviously climax so I suspect, if you are for real which you'd better be since I'm taking the time to answer you, that your problems are more psychological. Getting him to wank into a condom leads me to suspect that you think sex is dirty and wrong.
I strongly suggest that you see your GP and ask for counselling. Because yes, there is a good chance that your marriage will collapse. You see, one day he will get fed up of bringing you to a climax with oral sex whilst you do nothing for him. It's very one-sided and selfish. Right now he might put up with it because he loves you, but you are not showing him a lot of love in the bedroom are you?
I once saw something about the way men view sex on the TV and surprisingly many of them said that sex was their way of expressing their love for their partner. You are denying him this. If you said you didn't want any intimacy altogether I might have more sympathy for you, but you are happy to receive yet cannot be bothered to give.
Your relationship is not normal and therefore, for the sake of your dp, make an appt to see a female GP today and ask her to refer you to counselling. For your dp's sake.