My first response to this is that it's time to go into crisis control. The relationship is broken and it needs fixing.
Clearly S is not responding to your requests for more time, less sleeping alone and is, perhaps worst, breaking promises.
"He promises to sleep in the same bed as me at night and when I wake up and find him in front of a movie and ask him to come to bed he just gets annoyed with me." This right here is a Bad thing and looks like a good case-in-point for when you talk to him.
Out of interest, was S poly before this started? From what you've said, I feel he wasn't. Or at least, not very good at it. One thing I find can help is try to apply the issue into a "mono context". Swap out M for "the lads" (and sex for drinking) as the end result is more-or-less identical. He is excluding you for time with others.
If I was in a similar position (and I must admit I learnt this from techniques amaranth has used on me) I'd sit S down and say to him that this needs to change. Explain, clearly, what you want to be different. Then - and this is the most important - set him a deadline to fix things. 3 weeks works nicely as that gives him chance to change and then hit the tricky bit of staying changed and not rolling back to his current habits afterwards.
I also notice you're getting resistance from M. "He then comes to comfort me and she gets upset at him". Two big problems here:
The obvious one is that she shouldn't be getting upset at him looking after you. However, this is a small issue compared to the following.
He should not need to comfort you for having sex with his partner. If this is distressing you to the point of need comforting, he should not be doing it. They can go to hers, do it when you're out, have the decency to be quiet. There are plenty of options that won't/don't distress you. The two of them need to sort this out, but hopefully fixing things in other areas (namely, sleeping with you when he promises, rekindling your romance) should help this matter.
I wish you the best of luck and hope those two are as sincere about making this work as you are.