It's hard for them to 'get' though isn't it. I didn't get it either until I was actually pregnant. Last night I had to put up a pair of quite heavy curtains - it took boyfriend to hold on to the main weight of them, and then me on the ladder doing the hooking in. I had to take three breaks in 10 minutes because my arms hurt so much. Just getting upstairs to do it involved some puffing and panting too.
I think at the beginning they are a bit more sympathetic, since it's only in recent memory that you were your normal energetic/useful self. Now the 'real you' seems such a long time ago (seems ages to me anyway), that they think you've just turned into a lazy bint and it's permanent!
With mine it's the irony that is annoying. I heard him on the phone to his mum last night talking about how I was trying to stay active, and how he'd been encouraging me to go to the pool and to keep going. I'm just thinking fuck off! He hasn't been to the gym since April, comes home, sits on his arse in front of the telly with some alcoholic beverage and switches off. OK, actually he cooks dinner, but he'd do that for himself if I wasn't around anyway - and sometimes he potters in the garden. He's also forgotten that this is my summer holiday, that work hasn't finished for me yet, and that if he was on leave and I left him a massive list of stuff to do every day, he'd be pissed off.