Re open relationship discussion. I explained that I loved him, loved our relationship, would consider myself very happily married (all true), but there is a big libido mismatch. I said one solution I'd thought of was an open relationship, and then sort of took my cues from his response. It helped that he was willing to discuss ground rules and things like that, because it felt like if it was going to work then it would have to be a real joint decision, and one made on trust.
I also explained that if our sex life was more frequent I wouldn't be considering it - it's not about me wanting to have sex with other people specifically, and I think that helped. I mentioned, in a non-threatening way, that the lack of sex does increase the chances of me doing something stupid, as if I were to meet someone I could have casual sex with on the sly, I would be more able to think of 'rational' excuses for doing so - and I really, really don't want to do that. Of course, I could just not do that, but I do think it's human nature to want to be sexually desired, and if you're not getting that at home then it's harder to resist if you come across it elsewhere. And then of course there's a resentment that builds up for your partner, that would add to the sensation that deceiving them would be ok. Essentially, I made it about me rather than him, saying that I was concerned that either I would cheat or feel that I should leave, and that I would like to avoid both those things.
However, my DH is very aware that I'm unhappy with the frequency of our sex life, and does agree that it's not 'enough' - like I said, I'm not sure what the 'standard' is or should be, but he accepts it's a problem and is to an extent happy that I'm still here and want to work on it. If yours is calling you a sex maniac and claiming you should be grateful, you might need to spend a bit of time making it clear how much of an issue it is for you - he needs to take it seriously and be open to working on it.
On another note, SolidGold this - "These two myths tend to mean that when the libido mismatch is due to the woman wanting less sex, it usually is down to the man doing little or no domestic work and treating his partner as a domestic applicance with a fuckhole. When the woman is the one feeling unsatisfied and unloved, she has usually tried patience, kindness., gentle conversation, being 'nice', losing weight, sexy lingerie and bottles of wine, only to be either fobbed off or told she's a slut and a nympho."
I totally agree. I know men and women in this position. The women I know are ENDLESSLY trying to find ways to solve it. They're thinking 'I'lll lose weight' 'I'll make a nice dinner' 'Maybe he has a fetish' 'I wonder if he'd talk to the GP'. The men... really not so much. They seem to find it sort of unsurprising in a very depressing way, like 'well, of course she doesn't want to have sex with me' - as though they think, deep down, that no women do, really, and it often seems to lead to 'so it's fine if I cheat'. Occasionally via a detour of 'I bought flowers'. It's sad for them too, because it feels like it could be solvable. I guess it's how we're all socialised - women are raised feeling that men find them sexually attractive (often to a dangerous degree) and men are raised feeling that they might 'get lucky' and find a woman who agrees to have sex with them. There are good reasons for this, but also not so good ones, and I don't think it's healthy either way.