Only when she's high.
I dont think marriage is for everyone. I myself like the idea; its saying to your partner "hey, you, I love you enough to give you all of my shit and remaining years." Not an institution to be entered into lightly, and I see many couples doing just that. Easily in, easily out. Well, emotionally, anyhow.
Asking someone to marry you should be the biggest deal of the relationship. If you cant commit to it, dont ask. I know couples who are perfectly happy waking up every day and saying "yeah, I'll stick around" without any further ado- and you know what? If that's what works, great! The bottom line is this: If you commit, see it through. If you cannot commit, if marriage is not your ultimate expression of love for another, don't try. Better to leave the relationship than to half-ass a lifetime connection that you know will ultimately fail for lack of devotion.