He was surprised, and he didn't bother to cover that up. He got kisses from girls left and right all the time, especially when she was shitfaced, or he was shitfaced, or they were both shitfaced. She wasn't just a girl, though. No, that would be... might be a little rude, sure, but it just didn't describe who she was accurately.
She might not be the woman he wants to be sitting next to him on this bench, but... It wasn't a matter of 'oh, I guess you'll do' as it was being a different sort of comfortable with a different sort of company.
"Doesn't hurt to be happy every once in a while. Even if it is a cheap thrill."