What do you think is the relationship between MBTI type and sexuality--specifically speaking, orientation and sexual drive? Are certain types more prone to homosexuality, bisexuality, or transexuality then others?
It is a good question and worth exploring. Honestly, I am not sure.
My experiences with these groups (informally considered, I never took polls

) doesn't really show an obvious deviation from the normal MBTI percentages for type in the average population, as best as I can tell. Overall, SJs and SPs seemed to dominate, and N's were a minority, just like the normal population. Of those, NFs seemed far more common to me (perhaps overrepresented) than NTs.
More specifically, I have known ISxJ, ESTJ, ENFP, INFP, and ESFP homosexuals (ESFP seemed to be very common). Most transsexuals (pre or post) I have talked to have usually been either S types or NFs (a higher than normal percentage of NFs, I think). I honestly have trouble remembering ever meeting an NT transsexual (well, except for substitute.

)
Now that I am thinking about it, there is a small pattern there... just about all the transwomen I've met are F's, while just about all the transmen I have met are T's. (My guess here is that the T/F gender bias impacts self-perception.)
To make a broad assertion, I think SJ people would be most likely to fight to suppress/deny any minority inclinations, so they're harder to poll and are probably underrepresented if anything ... many are struggling to deny their inclinations and be "normal members of society." And Feelers have a harder time "faking things" for such a long time, where Thinkers are able to suppress their emotions and internal experience to conform, if they see the need to.
I can give more anecdotal evidence on this, if people are interested. (I don't have any "real experience" with self-proclaimed bisexuals, however.)
I can't answer the second question easily. ExFPs are sex sex sex (they seem to be into the pure sensation of it, a carnal "experience")... but xNTJs also have intense sexual craving more like a desire that needs to be fulfilled or an urge that must be satiated, partly as a means to express dominance/capability.
Then again, maybe I am analyzing this WAY too much.
From what I have been reading, NFs seek to transcend sexuality, even when engaging in the act, and SPs seem to revel in every experience of it. SJs schedule it and NTs intellectualize it, is the idea I get.
As a broad generalization, I would agree. And that's what you find when you read the subjective literature. SPs are VERY likely to revel in the experience of being gay or transsexual; it's all about the experience of responding to the desires inside of them. NFs are very much the ones who move past that, who are embracing their sexuality but moving far deeper than that and speaking in terms of "identity" and Becoming Themselves.