Morondela, I think people are generally quite perceptive when it comes to body language, the way people talk to each other, etc, so even if the couple aren't kissing, it can become evident pretty quickly that they are a couple.
I tend to notice women holding hands when I see it, and I feel like I can usually figure out whether they're a couple or just friends/relatives. When I'm with a date/girlfriend in public, men often watch us for a bit or discuss with their mates before they start hassling us, and even if we deliberately stop being so affectionate at that point to avoid their attention, our awkwardness about it all probably signals to them that we're gay and self-conscious about it anyway.
Whereas if I'm with a female friend, I don't feel self-conscious or vulnerable and I don't run through in my head things like "shit, did I kiss her a minute ago? I can't remember - I wonder if I did and they saw. Maybe they saw her stroke my hair? I can't recall exactly what we did because we were just being normal before we noticed the guys in the corner talking about us".