There's an opinion I've heard expressed about strategy for marriage equality, which believes that the loophole allowing trans people to be in legally valid same-sex marriages is a boon to be exploited by whoever is able to. I just cannot accept that.

For many transsexual people, exploiting that loophole would run counter to the whole thrust of their transition. For many of us, that compromise of personal integrity, especially after the arduous journey of transition it took to get there, to contradict our hard-won gender status in such a way is seen as unethical or dishonest. One's self-respect is too high a price to pay. Also, it betrays the cause of marriage equality by taking advantage of a loophole that isn't available to a cis gay or lesbian couple. For many of us, such a betrayal also destroys our self-respect, after having struggled in the trenches for marriage equality alongside cis gays & lesbians.

I've even seen an opinion that says cis gay individuals ought to marry transitioned trans people of the same sex; e.g., a gay cis man marrying a gay trans man. (Perhaps with the intention of subverting the system, or simply getting yours, scamming the system if need be?) This is proposed on the basis that the state considers the latter gentleman a woman.

Reality check: To enter into a marriage on the basis of the wrong gender that was coercively assigned at birth is just unacceptable for too many trans people. Cis people need to understand that our lives are not a game. Again, it isn't easy for trans people to establish their true gender; very many of us achieve it only at great personal cost in relationships, careers, and finances. After such a hard struggle, don't tell us to duck back into the painful, destructive place we just escaped from.

As to the argument that the trans marriage loophole can somehow help bring about repeal of DOMA, no it won't do a thing for the cause. Honestly, I just don't see how trans people scamming the system would soften the hearts of the religious right supporters of DOMA toward marriage equality. Rather, it would only harden their hearts further toward trans people. 
The legal status of trans marriages is a subject all its own, which is far more complex and harder to work with, and just as problematic as cis same-sex marriage. Trans marriage laws in the 50 states are a crazy quilt in a hell of a mess, due to the laws against marriage equality for gays. The two issues are linked, but let's not tangle them.

When I transitioned, my marriage remained valid in Virginia, despite being officially a marriage of two women. I'm not proud of the fact, but now I've separated and am intending to divorce. To make myself an honest woman, as it were. And then when I want to marry my girlfriend, Virginia won't allow us to get married because I'm legally a woman. Note that the validity of my marriage is not based on considering me “male,” since in Virginia they have me down as female. The only reason Virginia hasn't invalidated my marriage so far is that no such case has ever been challenged in that state. Therefore no law exists on the question, and loopholed same-sex marriages like mine continue by default, because they started out valid and… no one has challenged them, that's all. It would be to my personal advantage if such a case formed a case law that nullifies my marriage, because I want divorce though I'm poor and cannot afford a divorce lawyer. But that would be a very selfish thing for me to wish for, when it would mean that people who want to keep their marriages would have them forcibly annulled.

So the trans marriage issue is just too complicated and we'd be better off not even touching it. Any amount of juridical tinkering with it under the current climate of DOMA and anti-same-sex-marriage state amendments runs a risk of making things much worse for trans people. The way ahead to win full marital civil rights for trans people is to enact marriage equality for everyone. Trans interests and gay interests around marriage coincide here, and oblige us to ally closely with each other in this common cause for all our sakes.