why did mac choose tails, she did not want to marry
Sorry it took so long to reply... "JAG" has been off the air for so many years now, I rarely get questions anymore.
As for the coin toss in the final episode "Fair Winds and Following Seas,"
Mac chose "tails" as she had done on other occasions in the past
You can speculate she chose "tails" because she had lost when choosing "tails" in the past and so she hoped to lose the coin toss.
Anyway, Mac and Harm had decided to let "fate" decide and thus the coin toss. She chose "tails" and the ending showing the coin falling was undecided - some claim that because the coin was shown "heads" in the final frame that it meant they did get married, but that is speculation.
Apparently, the show's producer Don Bellisario was planning a spin-off featuring Mac alone in San Diego, which would have meant no marriage, but the show didn't come about:
See another recap of the final episode here:
Hope this helps.
Thanks for writing,