RootsMagic doesn't directly support same sex relationships, but here is somewhat of a workaround.

First if you have entered the partner you would need to delete him/her.

If you want to enter this situation for now you can do this: (Note: in RM 3, this may have adverse affects on other areas).

1. Do Add > Spouse
2. Select "Add new person"
3. Enter the info and click OK (you can't change the sex at this screen)
4. This will bring up the edit screen for the person and you can change the sex at this screen.