A few people have noticed the story of Jason Knight, Petty Officer 2nd Class, USN.
The military can force anyone in their legal purview to serve, gay or not.
This is an especially pernicious part of the UCMJ (as best I know it). Under certain conditions (stop-loss, etc.), they can fail to discharge or reject. In other words, discharge/refusal is their policy, right up until they decide it is not their practice, for the while.
Of course, this says nothing about the hundreds of judgements that the military has to make on people who claim to be gay but who aren't ...