Bars in Le Marais
"Le Petit Fer a Cheval" (30 rue Vieille du Tempe) is a good start. It is tiny, good to stand up at the bar and absorb the atmosphere - pretty mixed crowd. It is very central so there are other bars up and down the street. Also "Le Pick-Clops"(16 rue Vielle du Tempe) is pretty good, a little grungy in style but a good bar for a beer (quite young, mainly straight).
Obviously the bars range from pretty straight, mixed, to gay - it is pretty easy to tell so pick a bar and enjoy.