Jerry Seinfeld is narrating a story to George Costanza, sitting in the coffee shop, somewhat abashedly. During foreplay, his girlfriend was talking dirty to him, and when mentioned her panties, Jerry responded "You mean the panties your mother laid out for you?", leading to his partner ending the romantic interlude and leaving in a hurry. Jerry, hapless man that he was, was only trying to keep up, but said the wrong thing. As with all of Jerry's romantic partners, the woman is forgotten and the mishap never mentioned again, after the episode is over.
While it is nonsensical, the reason why it could be disturbing is pretty clear to me. While engaging in sensual moments, the mention of undergarments and parental figures in the same sentence as a pretty obvious Freudian aspect to it that I don't feel I need to explicate.
On the other hand, Jerry Seinfeld ran afoul of something that can often be a problem in romantic or sexual moments: while partners may desire spontaneity and freedom, it is also very easy to go overboard with such things.