There's a wrinkle I didn't know about or expect. It makes combat duty the plum, coveted job. The thinking seems to be, if you've got to be drafted, it's discrimination if you don't have your fair opportunity to be in combat.
I never thought of it that way, and I'll bet most people of both sexes don't view it that way.
Your thinking is a bit backward from the logic applied. The argument is that the draft is used, first and foremost, to fill the combat arms roles required in a major war. Therefore, if one is ineligible to the combat arms, one is not required to be in the draft.
From my memory (which may be suspect), the “include women” argument was really used as bait to try and get rid of the selective service altogether. The anti-drafters thought including women would scuttle registration. But their arguments were rejected because women cannot fill the primary roles sought in a draft.
The “plum role” part comes in the volunteer army, where combat arms are considered a distinguishing role, so that officers and enlisted who serve there have a leg up in the race for promotion.