Cheating and homophobia

So some of my friends and I were talking about how so many girls cheat. And how cheating can be seen as many other things, too.

I desided to hold a guy's hand who I'm not dating. And they told me I was cheating? I mean it wasn't in a "I like you" way it was a "Your hands are soft and they feel comforting" way. Haha. And also my boyfriend flirts with a lot of girls. When me and him break up he comments other girl's pictures and says "Yeah I'm single. I hate being single." But he tells me how much he wants me back. It seems like all he wants is someone to take the girlfriend role? And If he wants me back then why is he flirting? When we break up we both know it's not forever. Or for a long time. So my friends said it was cheating but I said it wasn't.

SO what exactly is cheating?

-Side Note-
I'm on phobialist.com and I was reading all the phobia's. I thought homophobia was just hating gay's. I realized it's being afraid of becoming gay. Or being seen as gay. It's also the fear of samness. So I've been hating anyone who was homophobic without realizing they could have a fear of many things and not just hate gays. I'm starting to feel extremely guilty.