Seems to me there are several issues
1 You don't see why it's an issue ,because you're not that fussed by sex.
There could be a lot of reasons for this, medical (more on that later), psychological (have you ever been the victim of assault or harassment?), Physical (to be blunt is he crap in bed? Are you making an effort? It does sound a bit like he's 'pumping away' and you're just lying there)
Well as I said in my earlier reply and several others have said - because sex and sexual intimacy, even the different way we kiss sexual partners is how romantic relationships are maintained!
2 poor communication - he's trying to tell you there's a lack of intimacy in your relationship yet expressing that poorly. You're not hardly saying anything to him about it at all!
3 couple things you've said make me think it's quite possible there may be a medical issue, I'm thinking hypothyroidism. Which can lead to difficulty losing weight, depression (which can lower libido), tiredness, irritability. Could also be undiagnosed pnd as you say things haven't been quite the same since you had your child 10 years ago. Couldn't hurt to have a check up at drs, it might be something as basic as anaemia!
I'd recommend you do 4 things
1 drs for some blood tests
2 read women are from Venus... or something very similar which explains a man's perspective on relationships.
3 go to relate or similar, they'll facilitate communication
4 'date night's are frowned upon in the UK for some reason but you need to do SOMETHING that's just the 2 of you, that's not sex related