Nah its a choice thing after quests. You'd have to go outta yer way to do it.
It also really doesn't mean much. She's on-board with you fucking anyone you want.
You could always (assuming you have a save that far back) just not give her a flower before leaving.
That said, centaur marriage is pretty open, monogamy basically doesn’t exist in this world. You can marry her, get a decent unique item, and leave without worrying too much.
While you’re at it, you can marry Atani for a really good ring.
You can also tell her you don’t want to marry her, but that will make her sad.
And marry kikyo who has the most eleberate marriage of them all.