Things have gotten … complicated with my best friend. I’ve known him for the better part of three years, and I’d personally consider him my closest friend at the moment. I’m a guy, by the way.
However, things are beginning to get complex between us. At least, I think so. Some context: He’s been dating a girl for the last few months, and they really love each other. However, my friend has very recently told me that, as well as liking women, he likes men, too.
Since then, he’s been … playful with me. Holding hands, accepting my back massages, sitting on my lap. Stuff like that. Today, when we were talking, he said he doesn’t mind me flirting with him in response, and that he’d probably say yes if I asked him out … if he weren’t already dating a woman.
I’m really uncertain at the moment. As it is, our flirting is just banter, but I find I’m putting serious emotion into it. I’m definitely falling for my friend. Is there any use in pursuing this, or should I leave it?
LemonVibe 
Have a heart to heart with him and tell him what you told us, but be aware he has already given you his answer which was “I’m already in a relationship at this time, but I like you.”
I wouldn’t wait around for him, but if later on you are both free then ask.
Feel free to remain friends with him though. Real friends, not “I have the ulterior motive of sticking kindness coins in you until you realize I’m the one and only for you.”
I agree with the commenter above. It seems like he’s playing with you, but has no intention of leaving his girlfriend. I think you’re going to be the one that gets hurt byt this, so a conversation needs to be had.