My guess is that propriety and reason bid Lady Montague to hold her husband back from fighting. Although she has few lines in the play, the lines Lady Montague has serve to prove her clarity of mind:
Thou shalt not stir one foot to seek a foe. (1.1.81)
First, Lady Montague holds her husband back because of propriety. They are in the city streets, aptly viewed by all. Her husband is quick to begin a fight, and all will see his aggression and his adherence to this silly feud. Secondly, reason holds her back. Being Romeo's father, I'm sure Lord Montague isn't as young as he used to be! Just as Lady Capulet calls for "a crutch" instead of a sword for her own husband, Lady Montague simply asks her husband to refrain. When one thinks of all the tragedy this feud causes throughout the play, it seems only to be the two wives who have any thought at all to decency (in public, at least). This might be sexist of me to say so, but isn't that always the way? Men fight. Women hold them back. It seems to be human nature. Then again, perhaps Lady Montague is simply a pacifist.