Well it depends on how you think your girlfriend will take it. I mean you can't just walk up to her or call her on the phone "Say what's up baby, can I come on you?", you know. If you two are good at getting into "private" conversations and the subject of cumming and sex and what not comes up then maybe you can bring it up. Id see it as no big deal if you came out and asked it during some sexual talk but I wouldn't want it out of the clear blue.
