Just about anything is now allowed on formal evenings in the DR. Even a jacket & tie seem to be taking a back seat to a pair of Dockers & Polo shirt. Lots of men like the more relaxed mode of dressing while still enjoying the better food in the DR. The women have always had it easier to conform, wearing a pair of slacks with any dressy top.
My husband just bought a pair of black jeans for our next for our next formal nights cruise.
Don't let that stop you from dressing up, if you feel the need.