Under the circumstances I'd be cool with it if I was Rick. I mean, come on, end of the world, your mate tried his best, thought you'd died (put his head to his chest and didn't hear a heartbeat) and then has the responsibility of keeping his best friend's wife and child safe.
There are few human beings alive who, plunged together in those circumstances, fighting for survival, who are both reasonably attractive, who WOULDN'T sleep together. It's just human nature.
I wouldn't be pissed about the situation 'cause it's understandable how things went down. What I WOULD be pissed about, however, is any CONTINUING relationship or shenanigans.
And I took it Rick was okay with it (or understood it) until Shane kept trying to keep the relationship going. Shane's biggest problem, always wanting what Rick has.