The problem, in Penny's head, wasn't so much the kiss itself but that she didn't see why she shouldn't kiss Roger (or someone else, for that matter). More than an adequate amount of time had passed since her relationship with Percy ended and Penelope was over it, for the most part. True, sometimes she found herself not quite comprehending why she had let him slip away. Even if it was his idea and she didn't hesitate too much to agree with him, it still felt weird for her to be single.
At some point, she was going to have to get over how awkward she felt just thinking about being intimate with another person. Even though her intimacies with Percy had never ventured as far as she would've liked...
But this wasn't that intimate. She was standing with a gorgeous guy and he was mostly drunk and really just wanted to be reassured that he could get any girl to kiss him, no matter how conservative and ordinary the girl may be. Penelope wasn't feeling pressured at all. It didn't feel like she had to do it. If it was something she was truly against, she would've said so right away.
So, predictably, she leaned forward to plant a kiss on Roger without too much more hesitation.