For a hetero couple, actually. You wait to get married until you can afford it.
A lot of people join the military while leaving their lovers back home. The military does not pay to have your girlfriend or boyfriend join you wherever you go, nor do they pay for you to go home on leave, regardless of the reasons. They will grant you vacation time, subject to job requirements, etc., but they do not pay for your trips.
Like I said before, the military doesn't owe anyone a spouse.
I'm not trying to sound harsh. This lack of nationwide marriage equality is a bad situation right now, but I don't believe that the military needs to be paying for service members to get married. Paying for a trip to a marriage equality state would count as a payment for a couple to get married, and that is not done for any military member. I can't think of any employer in this country that pays the costs associated with employees getting married, regardless of circumstances.