It must be particular confusing if a trans person is gay according to their mental identity.
Depends.
If the transsexual is seen by an observer as what they were born as, it might be a bit confusing. However, if they are viewed as their authentic gender, there wouldn't be any confusion at all. And that goes whether they are gay or not.
Not all professionals in the psychiatric field define sexuality in transsexuals as some in society do. Most do not. Some define sexuality in terms of the authentic gender of the person. Some define it in terms of the current genitals. Few define it in genetic terms, which is probably the most common determination made by the average person in society. For instance, I've noticed that many, if not most, of the people on this forum define sexuality as it relates to the person's genotype. So, if a post-op male to female transsexual has a relationship with a natal man, they view it as homosexuality. Few psychiatrists would agree with that.
What I find interesting is that those who would refer to that as homosexuality still would not approve of a post-op male to female transsexual in a relationship with a natal woman. That seems disingenuous to me. If they call one homosexual, how could they possibly NOT call the other as heterosexual?
But, every male to female transsexual I've ever come across that was attracted to natal women refers to themselves as lesbian, as would I if I were in such a relationship. Especially after surgery.
It is with an eye towards the controversy that would be caused for other Christians regardless with whom I was to have a relationship that I decided to remain celibate. It isn't Christ-like to knowingly do anything that can cause a weak brother or sister to offend. And since regardless of who I paired with there would almost certainly be somebody incited to be judgmental of my freedom in Christ, it is better to refrain. At my age, and with my history, abstinence isn't that much of a big deal to me anyway.