Am I the jerk for kissing my best friend's girlfriend while he was in another state for his job?

My best friend will call him John, that's not his real name.

Has been dating his girlfriend for around half a year.

His girlfriend never really talked but I knew of her because she was dating John

and he would talk about her all the time.

I didn't really start talking to her and spending time with her until the past two months.

John has been in another state for a job and he's coming back in August.

He wanted us to spend time together since we never really talked before

and he thought that we would get along.

As it turns out, he was completely right.

We started spending time together and immediately clicked.

I've always thought that she was a very beautiful girl but I could spend all day with her

just talking and not get bored.

Every time we hang out together, we spend 10 plus hours together

just exploring or talking.

We do talk about John sometimes too since we're both close to him

and she seems to like him a lot.

She also told John about how well we get along

and he was apparently pretty happy about it.

However, I'm definitely starting to fall for his girlfriend

and I think I already have.

I love spending time with her and I can't help but think about what could have been

...