he did not marry Tessa --
if you watch season 2 episode 4 or 5 (Darkness, i think is the ep title) -- he proposes to her --
according to a gypsie prophecy, Duncan was never married, and never would be --
as for you question - yes, his life in the 70's is something of a mystery -- i don't recall much of flashbacks dating to that time -
and the series begins in 1992, and Tessa, in the 1st episode, says they've been together for 12 years (she says dozen) - so, they would have met in 1980 -
there's an episode later that shows where/when they met - it was 1980 in Paris - Duncan was running from an immortal and leapt onto the tour boat that Tessa was working on -- describing to tourists the sights of Paris along the Seine --
so, the 1970's in the life of Duncan MacLeod, are oddly missing - i don't know why, unless the writers were attempting to aviod having some corny, cliche looking scenes of Duncan wearing polyester and trying to disco dance ------