partner changes mind about sex— what do i do?

i have been with my partner for almost two years now, and we’ve been happy and i do see a future with them. i have been asexual the whole time and i believed they were as well (they told me they didn’t mind sex but it wasn’t their priority). however, now they want sex and im unable to provide that. i’ve thought about opening it up to being poly but the idea of them having sex with a friend or someone we both know is eating me alive. i know there’s nothing wrong with me but at this age and the fact i still don’t want to have sex has made me feel discouraged. would love some relationship advice.