'Pregnant Man' Thomas Beatie -- I GPS'd My Ex ... But I Thought It Was Legal!

Thomas Beatie believes he had every right to track his ex-wife ... so says his current fiancee, who insists the famous "Pregnant Man" LEGALLY GPS'd the woman's whereabouts.

Amber Beatie tells TMZ ... after Nancy Beatie threatened to take off with Thomas' kids ... he secretly placed the GPS tracker on the car she was driving.

We're told Thomas did his own legal research and concluded GPSing her was fair game under Arizona law. He put the tracker on 2 years ago but she didn't discover it until recently.

Turns out he was right ... he was allowed to put the device on the car 2 years ago because at the time he was the registered owner.

But Thomas transferred title to Nancy more than a year ago, and at that point cops say keeping the device on the whip is plain 'ol criminal stalking ... which is why he was arrested.