why does my boyfriend (22M) suddenly want a threesome with me and my friend (both 21F)?

Gonna make this as short as possible. me and my boyfriend started off great sexually, such a good chemistry and so much love. i imagined we’d be monogamous forever, we’ve only been together a year and a half. for some reason he’s shown less and less interest in just me and my body when it comes to sex.

we still have sex, a lot, but whenever we flirt or dirtytalk he can’t help but bring up me letting him sleep with another woman, or us both sleeping with another woman. the part that rubs me off the wrong way the most is that he always bring up my girlfriends specifically.

It’s not just him bringing them up, but he’s explaining what he wants to happen in detail and want me to be as interested in it as possible.

He’s been a little unhappy, depressed, so i’ve tried my hardest to support this fantasy and act like i enjoy it even though my chest physically aches everytime we talk about it.

We’ve had so many serious talks about whether it’s a good idea or not, and i’ve said a few times i’m rlly uncomfortable with it and actually a bit hurt. he feels bad and promises me to never bring it up again, but then we have sex and he still brings it up like nothing happened.

Especially if he’s drunk. if i get visibly upset we just won’t have sex and he kinda, gets turned off and says we can do it another time. i don’t want this though because obviously i crave intimacy with him. so i always just go along with it like nothing. but when i asked him if he’s always had this fantasy he said it started after meeting me, which broke my heart because he dated his ex for years.

Just makes me wonder if im not good enough, if i don’t satisfy him. whenever i ask what i can do more of, even though i try everything, he says nothing and that he’s happy with our sex life.

Is it just me? am i not interesting enough, thinking of it and that he was monogamous in the past with no issue makes me feel sick