The thing is, as these answers show, there's no definitive answer. But I'd say the single biggest indicator of whether or not the SAHP is resented is how the working partner sees the job. If the working partner genuinely gets that it IS a job and that not having to worry about the kids, the mental load, the school run, holidays, childcare etc as well as often, especially when kids are older, not having to worry about cleaning/cooking/shoping etc... and on top of that, then sees all finances as being joint, then there's no resentment and it's all good.
The problem comes in when the working parent sees the SAHP as basically just having a good time. When the working parent doesn't see or appreciate what is being done behind the scenes. And when the working parent thinks he (or she) has more say in how money is spent etc. THEN you have a problem.
I've lost track on here of the number of threads from SAHMs where the DH is off on jollies on a regular basis, has expensive gym membership, goes down the pub with mates, plays golf etc etc but the SAHM can barely afford to get her hair cut because her housekeeping budget only just covers food, kids activities etc. And if he's challenged she's told, "But I worked for that money". It's bollocks.
DH and I 100% share money. I'm the working parent. Since DC are at school, he's started working part time, and truthfully, the money is nice, but I'm very aware that when he was at home full time there was a lot less for me to do.....