“On the British Isles, there are traditionally three genders: Male, Female, and David Bowie.” — Ruadhán, 2018, his Instagram

The notion that sex and gender are fundamentally different is a social construct going only back to the 19th century, and the identification of the "sex chromosomes."

Literally no-one is attracted to another person's chromosomes, and as literally all other traits of sexual dimorphism are mutable, well... Seems that any proposed definition of "sexual orientation" incorporating the notion of "biological sex" doesn't actually hold up to the real world.

To elaborate on the mutable characteristics of sexual dimorphism, gender-confirming medical care sought by transgender people, quite frankly, isn't unique to trans people.

• cis people receive HRT for various reasons
• cis women get breast implants, and the modification was invented for those who’ve had mastectomies
• cis men with gynaecomastia often seek breast removal surgery with results generally identical to that of trans men
• cis women who have survived cervical or vaginal cancer may receive vaginoplasty surgery as part of their treatment
• Partial phalloplasty surgery has been performed on cis men nearly eighty years, and complete phalloplasty a good decade before the first phalloplasty for a trans man
• metoidioplasty surgery is basically a modified version the surgery for correcting hypospadiac urethra in cis men born with said
• cis men who’ve had testicular damage that results in amputation (or in the rare cases of anorchia) receive silicone implants and HRT

Work with transsexuals, and studies of formation of gender identity in children provide basic information which challenges the notion that there are two discrete biological sexes. That information threatens to transform the traditional biology of sex difference into the radical biology of sex similarity… Every transsexual is entitled to a sex-change operation, and it should be provided by the community as one of its functions. — Andrea Dworkin, 1974 Woman Hating

The 23rd chromosome is literally the only difference between (most) cis people and (most) post-op trans people, barring any "intersex" variations - and many people with an intersex variation of the 23rd chromosomes don't even learn about that until at least puberty, if not much later in life.

As for pre-op trans persons...

The only reason to pre-emptively exclude trans people of one’s preferentially orientated gender (if one has a preferred gender) from one’s potential dating pool is socially constructed. The association of certain genitals with certain genders *is* a social construct, not a biological one — if gender is, indeed, a social construct, then it logically follows that so is the association of primary sex characteristics with a corresponding gender.

You think intercourse is a private act; it’s not, it’s a social act. Men are sexually predatory in life; and women are sexually manipulative. When two individuals come together and leave their gender outside the bedroom door, then they make love. — Andrea Dworkin (attributed)

Sex is a fundamentally social activity just as much, if not moreso than a "biological instinct" for procreation - if it were solely for procreation, same-gender attraction simply would not arise in any species that displays it (humans are far from the only species that engages in homosexual activities).

Sex is a social activity. Ergo, sexual attraction is informed by the social constructs of gender & "biological sex." (While the research into same-gender attraction shows compelling evidence for a neurological component, that doesn’t preclude the influence of the social constructs of the society one was raised in.)

These can be hard things to unlearn and relearn - much like how learning another language as an adult can be more difficult than learning multiple languages in pre-pubescence, but it's absolutely not impossible! It simply takes determination and dedication to the learning process.