Both Mildred and Clarisse, at one point, caught Montag's eye. He married Mildred, but later became good friends with Clarisse. So both women-at one point-interested him on some level. They were both beautiful in their own ways too. Bradbury says of Clarisse, "The girl's face was there, really quite beautiful", and look below for a quote on Mildred's beauty.
Also, both women were unfortunate casualities of the society that they lived in. Granted, their fates were on opposite ends of the spectrum: Mildred was the cold, miserable prototype of their stifling society: "Her face was like a snow-covered island upon which rain might fall, but it felt no rain; over which clouds might pass their moving shadows, but she felt no shadow". Whereas it was hinted that Clarisse, because her and her family refused to become as Mildred was, was taken away or removed (if her family wasn't fortunate enough to escape first): "And then, Clarisse was gone." So both women suffered in their society.
Those are just a couple ideas, and I hope they help.