Any bachelor party I've ever been to has been single sex. I think it's cause you want to say whatever and do whatever you want without anyone from the opposite sex being there. I think it's appropriate, single-sex parties. Cause the whole concept is supposed to be one last hurrah for the boys before you need to start dragging her around with you.
I would need to ask the other people of the same sex if anyone from the opposite could be invited. For instance, I'd want to clear it with the boys if a couple of girls who are friends wanted to come along. Especially if I was in charge of planning the party.
Finally, I wouldn't want any of the future Mrs. friends to be there. Guys or girls. Not because we are doing anything remarkable, but because its mine - she can have hers to herself as well.