He said he'd suggested they try a few things to spice it up and she had rejected him totally, getting quite upset. Now they were in separate bedrooms. They'd not had sex for ages. He didn't know how to fix the situation short term or long term. He loves her and he wants to have a sexual relationship. She's not interested.
Had he tried talking to his dw to see if anything is bothering her before his spicing stuff up suggestions? His dw may have got upset because he hasn't tried to find to find what's putting her off sex, and went straight to spicing it up. That could be a further turn off to her, and also feel like pressure. The language he uses "spice it up" if he's used same language to his dw then he may have come across as being bored by the sex they were having before issues. I can see why she would be upset.
I suggested they need to talk. He needed to apologise for offending her, explain how he felt and suggested that when she was ready they could talk about how to move forward. I said that he could ask her if there was anything she'd like to do that might get things going again. (Thinking that if it was her idea he wouldn't be offending her.)
They do need to talk, but if something is making her not want to have sex with him, they need to resolve that first. Instead of making it about what can I do to make you want sex, it should be, where did I/we go wrong rather than starting at a when will you want to do it? It may take a while though for her to be comfortable being intimate again. What did he want to do/say to cause offence? You don't have to say, but if suggested something the he knew she'd not be keen on, or role play type things she might find degrading, I can see why she'd be offended.
My DSis took a totally different approach, telling both of us that we shouldn't expect her to do something that she wasn't prepared to do and that she shouldn't be pressurised into having sex and that my DB should just suck it up if he really loved her.
Bus she also correct, he shouldn't be putting pressure on to her to have sex she doesn't want. I don't think he should suck it up though, There's every chance it an issue that can be fixed, but not by him pressuring to try things she does not want to,mor have sex with him when she doesn't want to. I guess it comes back to talking. I guess I'm saying he should be supportive and patient. If it's a problem that built up over time, it will take time to fix, so in a way, he should suck it up, at least for a bit, surely he would rather wait for a while, sort things out and have she wants?
DB got a bit angry and said how could she know she didn't like doing anything other than missionary with the lights off if she wasn't prepared to try anything else?
This is the bit that sounds iffy, because if he's getting angry at his sister for suggesting his dw can say no to sexual things things she doesn't want, is he being also angry/moody/sulky with his dw for saying no? I'd be in separate rooms too to be honest.
It also sounds like he didn't enjoy the sex they were having and if he's spoke to his wife the same way he did his sister about the sex they do have then that could be hurtful for her and make her less likely to want any sex, never mind spice things up sex.
She absolutely does not have to try anything sexual she does not want to, she can know what she doesn't want to do without ever trying it.
Eg, I have never had anal sex because I just don't want to. If dh pressured me and got angry when I said no and said I had to try it to know I didn't want it I'd be very offended and angry. If he kept trying it or mentioning it or being all grumpy about it then I'd probably not want sex at all.
I agree with everything acrossthepond said.
I went though a longish phase of being right off sex, I had a lot going on and I know dh wanted it but he never put any pressure on me, never complained, never sulked, I'd know I could hug him on the sofa with no expectations from him, he could see I was under pressure and helped me work through the stuff I had going on, which I really really love him for. I think if he started trying to spice stuff up, or complain about me only doing a b or c and refusing to try d e and f we'd not have worked through it, but he always said that the only sex he wants is sex that I want too, that he'd feel like a cunt having sex with me if he knew I was only doing it to keep him quiet.