Marines are much more likely to arrive in a combat area by way of transport aircraft, ships being the secondary method of arrival.
And in the 20th century, Cavalry arrives in armored cars or helicopters, having sold its last horses for meat in the very beginning of that century.
In some rare cases, there are tactical aspects to modern "Cavalry" that resemble those of horseback combatants of the 19th century. But much of the 19th century cavalry role is actually now performed by main battle tanks, while armored cars, helicopters and other such skirmisher elements are reminiscent of very different historical warforces. The term "Cavalry" is carried on despite
historical roots, not because
In that sense, space infantry might decide to call themselves Paras or Hoplites and actually hit closer to the mark than with "Marines".