If a transgendered person considers themselves monogamous and chose to be in a monogamous relationship with someone of the gender they are not attracted to it would surely cause some angst.
If they wish to be with a person of a gender different than presented by person they are currently in a long term monogamous relationship, they have several options.
1) Suppress these urges and remain monogamous.
2) Talk to their partner about their feelings and urges and IF the partner assents, transform the relationship into a polyamorous relationship. If so, one should be prepared for allowing one's partner similar privileges to act on their urges what ever they may be.
A GREAT and healthy example is the following:
3) Allow their partner to continue to believe that they are in a mutually monogamous relationship while in a surreptitious manner, arranging assignations with a party or parties that are in fact not the person that is agreed to be one's monogamous partner. Many people like to use what is often thought of as a judgmental label for this behavior. The label that is often used is "cheating"
Now, your question basically pertains to option three. I realize you didn't ask about the label, so lets leave that aside. As far as whether you should feel guilty, that depends on the definition of should. If you mean is there something pathologically wrong with a person exercising option three and not feeling the unsettling feeling that we know as guilt, then yes. In order to do so and not feel guilt, one either has a special natural ability not to feel guilt. This is becoming more common. Some of these gifted persons are"labeled" as having psychopathy of some sort. More commonly these feelings of guilt can be assuaged by any number of constructions and rationalizations.
Some suggestions that seem effective for some are: (in no particular order) "My wife (husband/SO/dog/cousin) doesn't really understand me", "I am a very sexual being and don't to burden the other person with such rigorous duty", "I am enlightened enough to realize that all feelings of guilt are just societies way of controlling my spirit which yearns to soar free", "This has more to do with personal fulfillment of a side of us that has been suppressed and subjugated, than with adultery", "When I dress, (or on Tuesdays and Thursdays, or when it is overcast, or when the moon is in full phase) I FEEL like a different person/gender/animal and therefore I didn't actually do anything wrong. There are many other possibilities, feel free to substitute your own. Extra points for creativity.