I have face to face knowledge of many individuals for whom that definitions does not fit at all, myself included. I am a well adjusted (according to me, at least) adult bisexual female. For a time I also experienced a great amount of needless anxiety that I think I could have avoided if I had transitioned earlier, I don't regret not doing it earlier because it's very clear I wasn't ready financially and emotionally.
I don't know how much experimentation the person you talk about did and if he would use the word 'regret' to describe what he feels.
Personally, I thought about transition regret a lot before starting even coming out to people. I think that if I ever decided to return to the point I started in the gender spectrum I would do it without regret because I would now have experienced it and felt that this life isn't for me, and I think I would do it with less anxiety. Not that I have any plans of going back but I feel more free to explore gender now and to not restrict myself to a particular box.
There are a lot of trans people attracted to people of their same sex, just like cis people. Who they prefer is a part of who they are. Jenner herself said "sexual orientation is who you go to bed with, gender identity is who you go to bed as" (paraphrasing). One does not affect the other. In most cases, an important part of a male sex fantasy is being a man and having a male body independent of whatever particular fantasy is occurring at the moment; the same case applies to women, women get aroused by the idea of having a female body when having sexual fantasies.
I quote a bit more
Blanchard’s model makes two etiological claims. The first is that cross-gender arousal arises from a “misdirected heterosexual impulse”; this claim was shown to be highly suspect in the previous section. The second claim—that cross-gender arousal causes gender dysphoria and a desire to transition to female in nonandrophilic MtF transsexuals—is based solely on Blanchard’s correlations and his anecdotal theory of “erotic target location errors.” No hard evidence has been forwarded to establish or support this proposed causal relationship. In fact, in the original papers in which Blanchard first developed and tested his theory of autogynephilia (Blanchard, 1989a, 1989b, 1991, 1992), there is no exploration or even discussion of the possibility that cross-gender arousal may be an effect of gender dysphoria (rather than its cause) or that both traits might simply correlate in nonandrophilic MtF individuals for some other reason. This oversight is surprising, given that Blanchard’s own research provides several lines of evidence to indicate that the causal relationship he proposes does not hold true. 
If you're interested in trans suicide there a whole PDF on it from which I quote:
In regard to timing of suicide attempts and gender transition, some surveys and clinical studies have found that transgender people are at an elevated risk for suicide attempt during gender transition, while rates of suicide attempts decrease after gender transition (Whittle et al., 2007; DeCuypere et al., 2006; Transgender Equality Network Ireland, 2012).