Well you said yourself, it's a massive deal breaker. So why aren't you breaking the deal? If he makes you feel sexually repulsive then that's not good is it? Regardless of the reasons and who is right or wrong, no-one should be made to feel sexually repulsive by their partner, whether intentionally or not. If you feel this way after only 18 months I doubt it's going to get any better.
Whether he loves you or not is irrelevant really. Some couples love one another very much and yet never have any sex at all any more.
Some people are sexually repressed and fearful of trying anything other than very straightforward intercourse, or they are uneducated about sex and don't now how to make a woman orgasm, as it's not always as straightforward as it is for a man.
Some people are lazy and selfish and can't be bothered to reciprocate, so long as they are getting what they need out of the occasion. Or they are not very emotionally intelligent about sex and they have a pretty narrow view of what happens in what order and they don't really explore anything beyond that, or question whether they are getting the better end of the deal.
Some people are properly grossed out at the thought of giving oral sex and just cannot bring themselves to do it. If this is the way they feel then it's very unfortunate for their partner, but it's hardly fair to pressure them into doing something they don't like or don't ever want to try . The problems often arise when those same people are quite happy to receive oral sex and look forward to it or ask for it. That can cause hurt and resentment.
I would be concerned that after only 18 months together he never initiates sex. And I think you need to be more honest with yourself about all these 'ups and downs' and 'external issues' you've been experiencing. From the way you talk I'm getting the impression that you are a great deal more invested in keeping the relationship together than he is, and I wonder if he picks you up and drops you when it suits him.