Hello, friends whom I haven't met!
I did not have a successful come out because of my mom's fallacious approach to my gender identity. Anyway, that's another story!  I know that if you have grumpy old parents, they gonna ask you 'till your heads blow up. It's okay, don't get mad at them. Calmly explain to them your views and kill off their curiosity (overstatement warning)!
As always, I will give a "sample" answer for you. I'm not one hundred percent sure this will nail down a deal, but you had better prepare to be awarded. I am not an SJW anyway, so save that for another day.
1. Original question: But that's not natural, that's not normal! 
    More than half of what you own is not natural: Your T-shirt, sofa, Vans, plates,... They are all man-made. But my sexual identity is not made of or created by anything; I am born with it.
    I am gay, a whole bunch of expertise substantiates my claim, and nothing is ever normal nor will it be out of the norm. You may have acne, scars, mental illness, small peanut, fat belly,... whatsoever, but you are human and trying your best to preserve your hominid instinct: to keep that strange gift for the rest of your life as long as it is not a disorder.
    (I think it's a gift, btw)
2. Original question: Oh my God you are gayyy?
Isn't gay mean happy? If so, I'm glad to be gay.
3. Original myth: Oh God! The surgeries are painful!
Oh, what an embarrassing verbal gaffe! I have just said that "I AM GAY", which means I am attracted to the members of my own sex. I am not a transgender. By the way, the pain is a non-essential part of being transgender since you can have the surgery or not at your comfort. If I am trans and think doing surgery worths it, the I'll do it probably happily.
4. Original dilemma: But you are our only son / daughter!
     I am not your son ever since I am 18, independent and care-free. But society has put way less pressure on me to pursue their rigid fucking stereotypes and social constructionist shits than you two. I am the only son, but not the only hope, and being gay does not mean I will be monsterized into some kind of cancer tumor. And if you are thinking about not being able to see my baby; adoption, non-penile fertilization, egg/sperm donation, and for the future,  in vitro gametogenesis. Stunned, right? Reproduction is not the only purpose of marriage. Am I right? RIGHT?
5. Descendants of the Ben "facts only care about my feelings" Shapiro: Your children will not have a MOTHER and a FATHER.
  At first, I have some instant thought about the issue. These are kinda half-truth since the sentence exploits an already existed stigma/prejudice on gay parenthood. Here is what can be said:
-Indeed, due to misconception, social stigma, and kinda-new-thing-to-society (it is not; until recently, many LGBTQ+ people lived in shame or secrecy, didn’t have the words to express how they felt or didn’t think that living according to their authentic gender was a real option. But that doesn’t mean that there were fewer transgender people out there. ) approaches to gay-ness, homophile pals' becoming a parent path is pretty rough. (Mommmm / Daddddd, how does the baby born?)
-However, the truth may tell a different story. Quote from the site: " Gay parents "tend to be more motivated, more committed than heterosexual parents on average, because they chose to be parents," said Abbie Goldberg, a psychologist at Clark University in Massachusetts who researches gay and lesbian parenting. "
Personally, I think we're damm good at this.
6. Anti-science jerks: You are probably confusing. 
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Thank you for being skeptical about my so-called "genuine self"
First and foremost, thank God cuz gay is a sexual orientation, according to WHO, a specialized agency of the United Nations that is concerned with international public health and a lot of science: https://www.who.int/bulletin/volumes/92/9/14-135541/en/
The American Psychological Association fucking nailed it:
“Sexual orientation refers to an enduring pattern of emotional, romantic and/or sexual attractions to men, women or both sexes. Sexual orientation also refers to a person's sense of identity based on those attractions, related behaviors and membership in a community of others who share those attractions. Research over several decades has demonstrated that sexual orientation ranges along a continuum, from exclusive attraction to the other sex to exclusive attraction to the same sex. However, sexual orientation is usually discussed in terms of three categories: heterosexual (having emotional, romantic or sexual attractions to members of the other sex), gay/lesbian (having emotional, romantic or sexual attractions to members of one's own sex) and bisexual (having emotional, romantic or sexual attractions to both men and women).
Confess to somebody who willl back you up first will help. - Tension Vab
Good luck, myhonies!
Ew dat's gay.