Agree with Zigner...
If nothing has been discussed with the male boss (which the OP has not confirmed yet) then there is currently no reason to believe, with the information given, that he is doing this to intentionally make his coworkers uncomfortable. There is nothing, so far, to suggest that he would not begin shutting the door if he were informed that it is making staff uncomfortable. All that would be accomplished by contacting a lawyer with the intention of claiming sexual harassment would be a huge legal hassle, a lot more discomfort/resentment/awkwardness etc in the workplace, and potentially some unnecessary negative financial and operational impact to all involved. Even with all that said, there is no guarantee that this will be considered illegal sexual harassment and if it is not, then there is also no guarantee that the employer/owner will do anything about it themselves. It also may put a target on the OP's back in the future.
Informal, professional measures should be exhausted first.