There's a lot going on in this and I'll try my best to address them in a coherent way. You're arguments seem to make several leaps and assumptions about attraction and relationships that aren't necessarily true in real life.
1) 'We can always tell'- One of the root assumptions of these arguments is that when people feel attracted to a trans person, they know the person is trans, and that's not always the case. Someone feeling attracted to a trans man is first and foremost attracted to a man. If a gay man is attracted to man, and later finds out the object of his affection is a trans man, he is no less gay.
2) 'Not being attracted to certain genitals means not being able to be attracted to trans people'- People are attracted to people, not just genitals. Initial attraction is usually based on other attributes (other physical appearance/personality/skills, etc.) before genitals get involved. I would go so far as to say most attraction in people's lives occurs without ever seeing genitals. You can fall in love with someone without ever having seen their genitals.
People feel attraction based on a number of factors, and when it comes down to it the people attracted to trans women are the same people who are attracted to cis women; the people attracted to trans men are the same people who are attracted to cis men.
Now, once you know someone has certain genitals you may no longer be interested in sex/a relationship with them, but that doesn't mean the initial attraction was fake.
3) 'Sexuality is what you do, not who you are'- If a gay man sleeps with a woman because of societal pressure or because he hasn't come out to himself yet, is he less gay for having experienced heterosexual sex? If a straight man is in love with someone who presents as a woman, who spends every moment of every day acting as a woman and uses a woman's name and she/her pronouns, you are saying that person is not straight if it turns out the partner was involved in a long-term deception. Nothing changed... except for the part where the partner lied about every aspect of their identity.
“Once you’ve been to Cambodia, you’ll never stop wanting to beat Henry Kissinger to death with your bare hands." ~Anthony Bourdain
Sooo, where does a transgender person, because of what stage they're at in transition, having parts incompatible with their love interest's sexual preferences come into play? As in a guy or girl won't date a particular transwoman, not because she was born male, but because she currently has male genitalia, and the guy is straight or the girl is a lesbian.
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