They don't generally check the names very closely (which in any case appear in very small print). Actual check-in is done using an automatic gate. You then have your passport only checked by a French or British border protection officer. As long as the passport is in order they will rarely even ask to see the ticket. You then have a quick final check of the ticket by a Eurostar staff member as you head up the stairs to the train itself. They of course do not ask to see your passport, so as long as the gender is correct they won't know a thing. Even then, their main priority is to make sure you are ticekted on that train, not what your name happens to be.