Enter domestic partnership date like marriage date

I added my partner to my tree but found that unlike a spouse, for whom I could enter a marriage date, I cannot enter a date for the beginning of our partnership. I find this odd, since our partnership is a matter of public record (I even uploaded the certificate to my documents), and will soon produce offspring, and so has some genealogical value. How can I reconcile this? I'd prefer not to enter her as my "wife," as I am striving for accuracy in my family tree.