"If you're certain that you want your startup, then your S.O. will read that and it's his/her turn to evaluate his/her priorities, and it might very well be that he/she will find the supportiveness for your endeavour now that you're dead serious about."

Or they will be pressured into grudgingly going along with it (and resent you for it), except at every small downturn where it seems you will fail (and I hear there's no shortage of those in a startup) they will, possibly in their very own passive-aggressive way, push you to give up--and when you don't, you'll just add to their growing resentment of you.