FH and I are getting married in a rather elegant mansion in the Fall. Several close female friends and relatives have mentioned that they're excited to wear a formal gown to the wedding, so I thought it would be best to put "black tie optional" on the invitation so that other guests who enjoy dressing up can without looking overdressed, but that no one feels obligated to buy a new gown or rent a tux if they don't have one because it is optional. Nice cocktail dresses and dark suits are perfectly fine in my opinion, but I know several family members will be in gowns and tuxes. I don't want to come off as pushy or demanding but I don't want people to feel over or under dressed.
Is it appropriate to include this request? And as guests, how do you usually feel about being invited to a "black tie optional" wedding? Do you feel like you're being told how to dress or forced into dressing a way you normally wouldn't?