When a man chooses to kiss the neck instead of the lips, it is likely to be an indication of sexual or romantic experience. Often it will follow passionate kissing on the lips.
It does not necessarily indicate sexual promiscuity or sexual playfulness verging on an unwillingness to be serious in relationships.
According to Cosmopolitan magazine, kissing anywhere but the lips may also be a signal of sexual desire. In other words, the man is likely baiting his partner into moving on to more intimate activity. Cosmopolitan recommends responding with a suggestion of something sexually experimental, such as sharing a shower.