I guess I don't understand what the problem is when one partner WANTS to make the other feel good. That isn't mercy sex. That's love. Believe it or not, we get a lot of positive feelings from making our husbands feel good even if we don't receive anything in the more traditional sense. It isn't a one way street as you guys are supposing.
Learning to accept a gift is a good skill to have. Calling it selfish to accept something that is offered without strings is mislabeling. You aren't being selfish.
I think, dobo, that it comes down to (and I'm stereo-typing) for guys it isn't getting off that we love most about sex. It is getting our partners off. So when our partner isn't, or can't, enjoy it, our prime motivation is gone.
I read in one of the Mars/Venus books:
When asked about their best sex experience,
women will tend to relate a time when their partner drove them absolutely wild.
men will tend to relate a time when they drove their partner absolutely wild.
So simply laying back, and enjoying receiving pleasure is not something that men are typically used to doing. It is simply a foreign concept to a lot of guys. Men, tend to keep score sexually, and if their partner isn't getting her's...then the guy is losing.
For OP, maybe your husband would react better if instead of doing something "for him", you tell him that he had better be ready, because when you're healthy again, the bill is coming due and everything that you're doing for him now will be paid back. In Full.
That also makes it clear that it isn't him, and it isn't really you, that is the problem right now. It is the medical problem that you have.