It really depends on the level of affection shown I think in my case.
Quick answer:
little kisses now and then and signs below that, are fine
Getting too open about it...ehhhhh not so much.
Gender wise, I don't think I have ever witnessed any case of it, though it might just have been me being clueless, but I think I'd probably be less tolerant if it was two guys. Not entirely sure why, possibly because I've always thought: "I don't judge you on your sexual orientation, just, don't shove it in my face too much".
So, that's pretty much how I can tolerate affection in public by various couple, just, don't be in my face too much about it. I usually have a hard time with cute little "blinded by love" young couples that feels they need to show their affection toward each other much more.