i don't have a problem with the word "cunt" but if i was having an argument with my husband and he called me that (or any name really) i'd be livid. i wouldn't expect that sort of treatment from someone i loved, even if we were mad at each other at the time. just like i wouldn't call him an arsehole or anything like that.
tis a bit different if you are angry or if you are just messing about.
like if i were teasing my hubby and he was laughing and said to me "you're such a cunt sometimes" then i'd just laugh. or most likely put on the fake pout and say "you're so mean to me, don't you love me no more".
if it is actually bothering you, ronnie ... i mean rah. then just say to her randomly that you're sorry that you called her that.