In my closer friendships I happen to be a very touchy person. I can hug my friends, hold hands with them... I generally show affection through different types of touch. This is where it might get a little confusing. Even though I am aro, I am open to dating someone if they want to, but it would probably be very similar to being my friend, with the only exception being kissing. I don't kiss friends, haha.
I guess for me the line is just the one action, because everything else that might be percieved as romantic are things I'm not interested in. In that sense, you might say the line is kissing, but I think the real line is just that I choose to call that relationship romantic and the other ones platonic. It's all about the mindset in my case.