If everyone were a homosexual, the species would become extinct. That simple fact forms a rational basis for discriminating against homosexuals (unless, of course, one does not give a damn about one's own species).
There are so many things wrong with this. For one, the premise isn't even true: gay people can still have children, just not with their preferred partner. Even if everyone was gay, I imagine there would still be many people who wanted children enough to do what's necessary to achieve that end.
More importantly, this is a ridiculous misapplication of Kantian-style reasoning. As Jason once put it (just replace 'marriage' with 'relationship'):
"But see, I don't for a moment believe that the principle behind gay marriage is to force all people into a gay marriage."
"I should hope not," she replied. "I like men far too much."
"It's a false derivation of principle, and exactly the problem with the Categorical Imperative. So all of this brings me to the other maxim that we might derive from the act of gay marriage, and it's one that I have to say I strongly prefer:
Let every couple be married who desires it, and let them spend the rest of their lives in a mutually supportive and faithful relationship, full of a deep, authentic, and abiding love.
This satisfies the Imperative quite nicely: Gay marriages and straight marriages both operate on the same principle, and this principle applies equally to all. If Immanuel Kant himself wouldn't necessarily have supported gay marriage, well, at least we might hope to bring around the latter-day Kantians."
Later in the thread Dallas responds to another commenter:
Dallas: I'm heterosexual and married, but my wife and I have chosen not to have children. If everyone were like us, the species would also become extinct. Should I be discriminated against too?
Yes. Selfish, narcissistic behavior should be discouraged, officially and unofficially.
Given how overpopulated the world is already, it seems a bit odd to call childless couples 'selfish'. Even more odd is the implication that one should have children out of a sense of duty. That's surely a recipe for disaster - what sort of a parent do you suppose would result?
I personally would love to have kids one day. But my motivation is not at all one of 'duty'. Rather, I genuinely want to have children (er, not any time soon, however!). One hopes most other would-be parents feel similarly. But given that we're just acting from our own preferences, isn't it a bit unreasonable to call those with different preferences 'selfish'? Yet this seems a fairly common attitude towards the childless (or so I gather from Butterflies & Wheels' old posts on the issue).
One can imagine (counterfactual) situations where perhaps it would be appropriate. If nobody wanted to have children, then we would have a genuine problem. If that were the case, then it would be appropriate to start talking of 'duty', as doing so could help us overcome the collective action problem. But at the moment, there is no collective action problem (or if there is, it's in the opposite direction)!
To highlight the absurdity, note that Dallas' logic implies that being an urban professional is immoral. After all, if everyone lived in the city, there would be no-one to grow crops, and we'd all starve.
But a strict adherence to universality neglects the value of individual differences and the benefits of specialization. Things will work out just fine if we accept the current diversity of preferences, and let people pursue their own ends as they see fit. So long as we have enough farmers, there's no need to admonish urbanites. Likewise, so long as we have enough voluntary parents to sustain the population, there is no obligation to reproduce.