Bob has no conscious control over which thoughts he picks up, and as such his power tends to be hijacked by his baser instincts. His Dirty Mind Reading is essentially a psychic version of Distracted by the Sexy.
Bob is reading the thoughts of hormonally-charged teens.
Sexual thoughts tend to be both "louder" than other everyday thoughts and pretty common.
Inverted: Bob uses his telepathic abilities to project sexual thoughts into others' heads.
Subverted: Bob reads something which sounds absolutely filthy, but turns out to be Innocent Innuendo.
Parodied: Bob ends up grossed out from hearing thoughts involving literal filth and poor hygiene habits. It works on a perfectly innocent level and is a Parental Bonus.
Zig Zagged: Bob has grown up with this power all his life, and no longer finds it remotely embarrassing when he picks up on stray sexy thoughts. However, he's still human, and when he's in the mood he tends to idly surf around until he finds something juicy. When his friends find out about this they're horrified at the invasion of privacy, pointing out that it's almost literallyMind Rape (or at the very least Mind Voyeurism) Feeling guilty, he suddenly becomes very aware of people's more intimate thoughts, which just makes him pick up on them even more often.