This is just playing with connotations. A bachelor is an unmarried man, so one could say that Tom acts like a bachelor despite being married. He is not a bachelor, though. To show this has a practical implication, assume Tom met Mary: the two could not get married immediately. If he were a bachelor, they could. He therefore lacks necessary properties of bachelorness (most significantly, not being married), and cannot be a bachelor, even if he may live his life much as a bachelor would.