We don't sleep together / we rarely sleep together
There's no passion / thrill
I would leave, but she has depression / any other long term illness without a clearly defined progression
I do care about her, but we're more like brother and sister
She isn't interested in doing anything, hasn't been for years
She's the mother of my children, she'll always be special to me (not so special I won't cheat on her, though)
I've tried so hard to make it work
I just feel like my life is going nowhere
And of course, all the same old flannel that men have always used to get insecure women into bed:-
"You're so sexy" (if she thinks she's not)
"You're so intelligent" (if she thinks she's not)
"You're so interesting" (...)
"I've never met anyone like you before" (he has)
"You're an incredible woman" (especially is he is 20 years older)
And then the things he says which may actually be true:
"I just can't help myself"
"I can't offer you what you deserve"
"I'm not good enough for you"
"I can't believe how lucky I am"