It's impolite to criticize your friend's dating choices.
Dave and Mark are talking when Mark tells Dave that he asked out Krista, and she said yes.
Dave wants to react to Mark's story.
Dave tells Mark that he's happy for him, and he wishes the two of them a happy time together.
Dave tells Mark that Krista slept around before and that he doesn't think Krista is really the right kind of girl for him.
Mark feels supported by his friend and is glad to have someone to share his joy with.
Mark feels disturbed that Dave knows Krista's sexual past and feels angry that Dave is judging her without knowing her.