Can't swallow cum

I cannot for the life of me swallow my boyfriends cum. The past times we've tried I've almost puked. And he's been eating fruits now and wants me to try again and I want to too but I'm scared that it's going to taste the same and that I will actually puke this time. I can't even taste pre-cum without wanting to puke. Is it my gag reflex?? How come I can't?? Any tips or facts as to why is appreciated!! Thank you!!