Really don't, because I'm beyond sick of it
I know, I know, you want to make it clear you're totally not a homophobe and it's not about the same-sex couple you just don't like PDAs - of any kind!
But the ONLY time this gets trotted out, certainly in any large numbers, is when LGBT folk have shown the same public love for each other that cishet folks get to do repeatedly every single day.
I do not believe you decide to express your distaste for PDAs every time straight people kiss. I do not. Considering the saturation of straight affecting in the media you couldn't even read your toddler a fairy tale without having to tut "oh look at this PDA, how inappropriate" under your breath. If you turned on a television you would lose your voice having to repeat your disapproval so many times.
There is absolutely no way you express your disapproval of PDAs every time you're confronted by straight people kissing. You would be on permanent repeat, like a record that's skipping.
You don't comment on straight PDAs but you hone in on the MUCH MUCH rarer same-sex PDAs to speak about? Yeah, that's some not-very-subtle shenanigans right there
And if you are genuinely unhappy with any PDA and this totally isn't isolated - then consider whether you need to express this NOW (because you DO let straight PDAs pass, don't even try to claim you don't). Even if you are super duper sure that you are totally not a homophobe in any way, shape or form, be aware that you do sound like one. Be aware that you are speaking in a context where a gazillion of your fellow straight folks constantly use such weasel ways to be homophobic in the hope they can wave the hate flag without backlash. Be aware that there's a whole bunch of straight people with unchallenged privilege and unquestioned prejudice who register, without even realising. same-sex PDA as obscene and needing reaction but don't even notice straight PDAs because they're background noise and you sounds a whole lot like them. Be aware of that "without even realising" and ask yourself how sure you are you AREN'T one of them.
Be aware that, to me and many other LGBT folks, you're part of a vast sea of straight people who've decided to express their disapproval over our relationships. Again.
If you quack and waddle, how sure are you that you aren't a duck? Even if you aren't, you can't be surprised when we reach for the orange sauce
And does it suck that you're totally-well-meaning-and-not-homophobic-
This also applies to the "I think all marriage/adoption/surrogacy/IVF is wrong" but only say so when we're talking about same-sex couples engaging in them crowd as well.