very new to poly
so i am about 5 months into a new poly relationship with an already established couple (they've been together 7yrs) and i am finding that this is way more confusing than i first expected. i started dating G first. she and her husband were very upfront with me in the beginning and told me exactly what they were looking for; a girlfriend that they could share. well the first month with her was amazing, she was extremely affectionate and was always telling me how much she couldn't wait until we were together. the sex was amazing and it seemed as though we couldn't keep our hands off of each other when we were together. once the three of us had started our relationship the physical contact from G started to lack. at first the sex between the three of us was constant but because of one of the rules of theirs was that S and i could not have sex unless G was involved, i have not had any sexual contact in over 3 months. S and i seem to have gotten closer throughout this time of "drought" for me and has been open enough to tell me that he and G have sex at least once a week. this is fine, obviously they are going to have sex, they're in a relationship. my problem is this... When i ask G if she and i can have sex she tells me "I haven't been in the mood for sex lately"... this bothers me because i know she is having sex with S. I have tried to have conversations with her about it but they seem to all just come back to the same end. she says she loves me and wants to be with me. i have been really fed up and ready to throw in the towel because lately it seems that all we do is argue. i am not sure what to do. does anyone have some advice?