This is clearly not a medical fetish issue, but rather, a relationship issue. To me, he sounds insecure at the least..possibly even immature. If he cares for you at all, he should know better.
First of all, he should respect your privacy by not bringing your fetishes up to anyone publicly...period...EVER! That's just not cool at all!
Secondly, he should be sensitive to your emotional condition when you're concerned about a sick friend. In that situation, you need a shoulder to lean on, not your boyfriend accusing you of looking for some type of sexual gratification in your friend's suffering....THAT'S TWISTED! (his accusation, I mean)
I think Dr. Phil would opine something about his repressed feelings of inadequacy and thinking he cannot satisfy you because he's not turned on by the same things as you...yada, yada, yada. Not to sound like a relationship counselor, but I'm curious how old you both are & how long you've been together. To answer the last question in your post - you shouldn't feel obligated to make him understand...it's an issue that he has..you've done nothing that needs explaining. He, on the other hand, clearly needs to explain to you why he feels the need to bring up your private sexual desires in front of your friends. He's the one with the issues, sweetie - not you.
Good luck & big hugs!
Brian (who's 38 yrs old & married for 3 yrs, by the way)