Well, I can tell you that if this were me I would ask for some firm boundaries on NOT having sex until you two have figured out what is going on that sex is on hold.
Sex comes and goes it seems. Sometimes in a long term commitment there is little and sometimes there is a lot! I have been married 10 years this summer and have noticed this over a good deal of time. You two however are only two years together and the sex has ended! That is not very long to be in that ebb and flow I don't think. People are different though.
I see this situation as a big red flag actually... I am wondering how much of her movement towards poly and consequently this new partner is to do with her being poly and how much of it is to do with needing an out of the relationship she is in with you. Sometimes partners become friends and there is no real reason to break up other than it just isn't a romantic relationship any more... something to think about anyway....
The date night idea is a really good one! Easing back into sex might be all it takes over the course of reconnecting. I think I would ask that she has a bi-monthly date with this other person or even monthly for a few months or more until the two of you have figured out what is going on between you both.
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