Hi KP =)
Has he always been this way? Sort of. It's just got much worse in the past couple of years.
Has there been a change? We got married... Although it started way before that.
Does he have the same attitude to mess in other situations? He has the same attitude towards anything gooey, sticky, icky or yucky (his words).
What have I tried? In the "non-kinky" realms I've tried the whole "sex is a no-no" for a while (based on you always want what you can't have), plenty of intimacy (kisses, cuddles, stroking, etc), setting the mood with candles, music, light meal. Through to light bondage, anal play (for him), being submssive, being dominant, dressing up (one or both), role playing, plenty of toys (most chosen together).
What works one time often doesn't the next - bit of a lucky dip situation.
That sounds like a really tough problem, and I appreciate that you've been working at this really hard for a long time with sporadic results.
Based on what you've said, I am suspecting that your husband has a strong association of "gooey, sticky, icky yucky (his words)" with sex. That sounds really problematic because how is he supposed to get in the mood for something he thinks is icky and yucky? Until his outlook about sex changes from "icky yucky" to "scrummy yummy", I think things are likely to go on as they are, unfortunately.
My opinion is that this is a problem you are unlikely to be able to sort out alone. You've tried for a long time, with little improvement. From what you've said, it sounds like his interest in changing his outlook towards sex is limited. And I think that for any change in his outlook to happen, he is going to have to want it to happen. Without that, I think any change is unlikely.
So I think that your best bet is to find him an appropriate counsellor who can help him to realise that sex is a delightful part of life with body secretions contributing deliciously to that. If he wants to address tis problem, he will help you do it. If not, I think there's nothing you can do.
I wish you all the luck in the world.