Tbh it’s not just kissing, them just holding hands or speaking affectionately towards one another is enough to set him off. Plus we gotta remember it’s been more than a year since they got together, I think they are past the metaphorical honeymoon stage? I mean I realize they are ridiculously happy with each other, but I think that’s just Kataang in general.
I can totally understand Sokka being grossed out if his sister was making out in front of him, but that’s not what sets him off. It’s just them being a normal couple and I think Sokka’s reaction is just a bit OTT considering how he was in ATLA towards them.