My girl doesn't like for me to cum in her mouth either, however sometimes tolerates it. This is now beginning to be a more regular thing for her since blowjobs are fairly special occasions for us.

We talked about it first, which is definitely the most important thing. She said that she doesn't mind if I let her know I'm close so she can prepare. Then, she usually will stop once I've started cumming and finish me off with her hand. This is a great compromise, as I still have the pleasure of going with her sucking me but she doesn't have to swallow the load and try not to gag.

Good luck!