If homosexuals are comparable to infertile heterosexuals, it is a disability.
Ignoring the subtle ableism here, which is almost certainly beyond the likes of this person, why not compare them to childfree-by-choice people instead? Which isn’t quite accurate either because gay people occasionally do have kids. Even then, I’ve seen people try to spin childfree as a mental illness…
If homosexuality is comparable to straight people engaging in oral or anal sex, it is a fetish or paraphilia
Straight people engage in kinks and fetishes too. And as these things go, those are pretty mainstream - there are far more straight people who at least occasionally engage in anal sex than gay people who exist, and it’s pretty unusual for straight people to not at least try oral these days.
If homosexuality is comparable to friendships between those of the same sex, why do they have sex?
Straight people (and for that matter, any people) with healthy relationships are usually friends with their spouses/partners. The concepts are not directly related but hardly incompatible either.
If homosexuality were as safe as heterosexuality,
In brief: “More susceptible to disease” is true in the very narrow sense of m/m reckless casual sex (because men transmit STDs more easily than women, and men are more likely to be into reckless casual sex) but this relies on a largely outdated stereotype *and* ignores lesbians. “Damaged sphincters” is not a thing for normal anal sex, and extreme stretching isn’t a common fetish regardless of sexuality. “Bug chasing” is a fetish for straight people too. “Refusing to get tested / not inform partners” is a straight people thing too. Unlike Covid, STDs aren’t airborne.
If homosexuality is a valid form of romance,
It’s not, it’s a sexual orientation. Heterosexuality isn’t a form of romance either.
…Which is an advanced-level discussion that would require too much space to cover even the basics of, so I’ll just note that the part that follows this is really invalidating to straight people who are childfree, unmarried by choice, or don’t believe that “until death do you part” is a realistic prospect for most.
If not being able to get married was a reason why homosexuals were so promiscuous before, why has it gotten worse since then,
It hasn’t.
with the degenerate mainstream media becoming more emboldened in promoting lifestyles like polyamory and cuckoldry?
You’re trying to link this with the previous statement, but it’s only tangentially related, in that being more open in showing one form of “alternative” relationship in the media tends to lead to being more open in showing others as well. Also, straight people do poly, and cuckoldry is almost entirely a straight thing.
Why do so few express interest in actually getting married, or treating it like the strict, closed relationship it is supposed to be?
Because you only hear from the ones who exclaim such views. This doesn’t make it the majority. This has been often noted on in BDSM communities: You’ll see a lot of people talking about poly (either in general or one’s own relationship), yet most BDSM people aren’t actually poly - nobody talks about being “not poly”. This creates a weirdly warped and sometimes uncomfortable perception for newbies. Also, straight people do this too.
TL;DR - OP has an overly narrow and inaccurate view of straight people, uses said view to attempt to invalidate non-straight people, also a few PRATTs, outdated stereotypes about gay men, ignores the existence of lesbians.