I know how you feel. I used to have a very strong aversion to fish, the smell and the taste. Once, I was at a school function banquet at a nice but very small restaurant- before the banquet you could choose vegetarian or non-vegetarian options, and since I'm not a vegetarian, I chose that. Imagine my surprise when they brought out the first course, and it was not a salad, but rather a plate with two of the biggest sardines I've ever seen. They were HUGE- I didn't know sardines could get this large but that's what everyone else at the table was saying they were- maybe 7 inches long each. Head on. Eyes in. Glistening scales. Plus, with everyone else at the table eating them, the smell was overwhelming. I had to excuse myself. I thought I was going to vomit at the table. I tried to be very delicate about it and just said I was going to go outside and smoke without saying anything about the food, though I did indicate to my friend next to me to have the waiter take it if he came back.
That said, I do think people are offended when you indicate their food is unacceptable to you. Chicken thighs are good enough for us, why aren't they good enough for you? type-thing. Even though I know that's not what your issue is, it's not the "quality" of the meat, I think people and maybe your in-laws could see it that way. Of course, fat and cartilage are not things that most people enjoy actually eating, but maybe you could practice spitting it out and moving on? Though I still wouldn't want to eat one of those head-on, whole sardines, I have started making myself eat seafood on a regular basis because it's so low-cal and good for me and I felt like there were social situations where I felt like a baby for saying I didn't like it. Especially at first, it wasn't my favorite, but really, I eat a lot of stuff that isn't my favorite. Not everything I eat has to be the most delicious thing ever. And the more you eat it, the more you get used to it, and now I even really do LIKE certain kinds of seafood (though I know you said you're allergic, I'm just using this as an example). Maybe you could just practice with some kinds of meat before you are confronted with a plate full at your in-law's. Not to say that you'll love dark meat all of a sudden, it's not my favorite either, but there does seem to be a lot of incentive for you to be able to at least stomach it.
