1. Severe allergies actually would disqualify someone from military service, so on that point you're proving your opponents right. 
2. The military cannot select for "potential divorce at some point" because literally everyone would be excluded. The military can only do so much to include and exclude people and still maintain necessary numbers in the ranks. This is why nations in times of war tend to relax the requirements re: weight, age, criminal record etc. 
2. You propose making accommodations for trans soldiers going through tough times - duty reassignment for "appropriate" (i.e. easier) duties. Well, maybe that's an accommodation the military doesn't want to bother with because it doesn't need transgender individuals in the first place to meet recruitment targets. This is about reducing risk, reducing cost, increasing efficiency and battle effectiveness, etc. It is not about accommodating people unnecessarily.