I love these thought provoking threads.
1st, I don't think we can have an expectation that transgender people are going to be any better than non transgender people overall. We are 1st and foremost humans, the good, the bad, the ugly. Half of all marriages at least fail. A big reason is infidelity. Is infidelity on the rise, or are people now holding themselves to a higher standard and choosing to not live with lies and a cheating spouse? My bet is the latter. So, in a way, that half fail number isn't necessarily a bad thing. IMO of course.
As for having a friend that is lying to their partner/ not divulging the truth of their being transgender, and going farther and farther on the path, a good friend, a true friend can only advise them of how they are only trapping themselves in their own prison by doing so. They can tell their friend that a marriage built on a weak foundation of lies will eventually fail anyway. You can lead a horse to water, but you can't make them drink.
A true friend will be there for them no matter what, once the bell tolls. Support them with whatever the result may be. A true friend doesn't have to give up their own standards, or cover for the other by way of perpetuating lies. We can draw our own lines as far as our behavior goes. Love the sinner, hate the sin.
What I often see on these forums is someone who will hit the jackpot of an accepting partner, or better, one who prefers the alternative lifestyle of gender variance. That person will not understand how or why others have such difficulties in their relationships. They misunderstand their behavior to be superior which causes someone to accept or like their gender variance. No matter how much a person makes sacrifices or compromises, an unaccepting partner is simply that. And, there are still a great many out there who are unaccepting.
The same goes for those who live in places that have higher than typical acceptance to gender variance or same sex orientation. "It is all in your own head, the fear of being out and about, no one really cares", so says the person living in San Francisco, or Greenwich village, or Provincetown. As accepting as those places are, there are places that are every bit not accepting. People are still being murdered just for being gay or transgender. People who are gay or transgender are still committing suicide because of the lack of acceptance from those around them. Their family who disowns them, their partners who leave them, friends who abandon a life long friendship because of it.
Like others have said above, the fear of abandonment, of being alone is strong. It is not always such an easy choice, to be who you really are but alone, or to be together with someone you love and hide. Of course we know that the best partner to have is one who knows all and accepts all. And the best way to find that person is to tell them upfront. Then, We don't have to go through all of the work to get acceptance, the often times years of patience while their partner slowly builds a comfort level to a point where they can even accept that their partner is transgender, let alone be ok with whatever changes they make in their lifestyle. Especially a full transition to their internal identity.
Obviously, there are success stories on here and other places. Some marriages and partnerships do survive it. Many don't. Anyone who is still concealing the truth of their being transgender lives in that fear that their marriage or partnership will end, and there is a good chance that they are right too. I think some people adopt an attitude of continuing concealment thinking each day I get away with it is another day with them and one less day without them.
Obviously for those who are or will transition, the bell tolls eventually, as it has to. Helen Boyd who most of us are aware of has even said that in cases of those who are CDers (not transitioning) that the wife not knowing may not always be the wrong thing. I am not going to necessarily agree with that. But from someone who has lived with it to the point they wrote books about it, their opinion is worth at least a few grains of salt.