I have never met a transgender person and given how few their numbers, it's possible I never will.  
We hardly know anything about them. There  are only rough estimates of how many and no one seems to have a clue on the breakdown between transgender men and women.  We need to learn more.   If there are men trapped in women's bodies that want to serve in the military I say maybe that's OK including combat duty.  But I'm not so sure women trapped in men's bodies.  There may be a role in the military for them but I'm doubtful it's combat.
It also seems that hormone therapy causes  psychological changes which given the nature of combat doesn't sound like a good mix but maybe not. Are some transgender people only joining the military to get their therapy paid for?  Perhaps only those that have successfully transitioned and pass the  psychological screening for the military (suicide and disposition to violence) should be allowed in.     I'll say it a second time. We need to learn more.