Probably best to let the guys answer this, but here is my observation...
Most guys see a woman they find attractive who has a penis. Something "clicks" for them and they explore this more. For most guys it starts as a purely sexual impulse and curiosity. For many guys it firmly stays that way.
For other guys, they develop an appreciation for the women of their coriosity. They become attracted not only to our physical bodies, but also to our strength, courage, and determination. They find that we posess an insight into what it is to be a MAN as well as an understanding of what it is to be a woman. The non-sexual "best of both worlds".
Pleasuring one's sexual partner is important. So, sucking on some "sheinis" seems elementary to the guys who want to date/sex-up a T-gal. They view the activity not as "sucking cock", but as making sure their girl has a good time too.
As far as the comments on how some of the gals look.. that's really not fair. People are attracted to what/who attracts them. One man's goddess is another man's fugly. So, you may see a "man in a dress" (
) where another man sees the perfect compliment to his particular taste.
Sexual expression is 100% beautiful all the time between consenting adults. Man/woman, woman/woman, man/man, man/trans, woman/trans, man/woman/trans/man/man/woman. Personally, I reserve "disgust" for pornographic depictons of rape. I may find particular "players" in a sex combo "disgusting" (Ron Jeremy), but the expression of the sex doesn't disgust me.