Sorry, this will be long!
I cannot believe that a few posters are trying to suggest that men aren't very good at understanding their partners' signals. What could signal NO more than their partner being asleep, ie unconscious?! Or that he was just trying to be sexy and got his signals wrong! Touching your partner sexily/lovingly whilst they sleep to see if they wake and fancy a bit of action is one thing (though I find that a bit creepy, whoever's doing it, but each to their own), lifting your sleeping wife's nightie up to shove your cock in her vagina when she knows nothing about it, can't say yes or no and especially when you haven't done one single thing to make sure she's feeling arousal too is sickening, wrong and 100% unacceptable.
10 year old girls get wet, 100 year old nuns get wet, women in comas get wet, hell, fresh corpses can have wet vaginas - does this mean that all those females are wanting sex?! It's absolute nonsense! By his own admission, this man thought that the only sign of consent was vaginal lubrication. Are we supposed to believe that a grown man, who's fathered 3 children knows so little about basic anatomy and about the reasons why a vagina would be wet?! Don't make my arse laugh!
I asked my DP for his opinion on this and read the OP out to him - he crinkled his face in disgust and said, 'Why would anyone want sex with someone who's asleep? How could that turn you on?'. He then got a bit upset and mumbled and when I asked him what he'd said, he replied that he wished I'd not read it out to him, as it had never occurred to him that someone would do such a thing and it had really disturbed him. BTW, I am very ill in pregnancy and we haven't had sex for 3 months and he hasn't complained once and he has a high sex drive. A man who respects his partner, loves her and cares for her needs and feelings would never do such a thing in a thousand years, no matter how sexually frustrated, rejected or confused he was. It makes me worry about the relationships of the women on here who think this wasn't a big deal!
OP, what your DH did was attempted rape. Rape does not have to be a massive, terifying, violent act or even have criminal intent behind it - it is, quite literally, penetration without consent and that is what he was attempting. Was his intention to rape or assault you? No, it doesn't sound like it. But it does sound like he doesn't care very much for your feelings and wishes and he certainly doesn't respect your body and that is a very big problem indeed. After many years together and 3 children, one of whom may be SN and is v demanding, it's pathetic, immature and disgraceful for him to be whining about you not having the same sex drive now as you did when you first got together. And 'I'm sorry you're upset' is a bullshit passive-aggressive non-apology, meaning 'I've done nothing wrong, but you're upset for some reason, so if I apologise will you shut up about it?'. There's times when everyone want sex but their partner doesn't, male and female, but you either drop the subject or go sort yourself out. You're not his personal wank-sock and the fact that he can't see what he did wrong or feel genuine remorse is very worrying. I do think this sounds like a final straw, from what you're saying and that counselling is a must.
Finally, listen to your own instincts. It is your relationship and you know what the boundaries are within it and if they've been crossed. You clearly feel like this is unacceptable and a violation and that his inability or refusal to accept that he was wrong and that he's upset you points to a deeper malaise within your relationship. If that's how you feel, regardless of whatever any of us say, then you must be true to yourself. Good luck.