If you’re libertarian, one of your core tenets is the value of the individual and the need to protect the rights of the individual – and sexual harassment undoubtedly represents an infringement upon the rights of an individual. You may have less of an issue with adultery between consenting adults or even with prostitution (freely-agreed contracts!) but ultimately whatever happens must be agreed upon by both/all parties. Cheating on one’s wife and humiliating her in a public scandal isn’t usually part of an agreed contract. (Someday we may have a political power couple in an open marriage, and it will be interesting to see what the public reaction will be.)
However, modern liberalism usually defines the world in terms of groups and group rights. The rights of the individual are much less important (see how often the Left criticizes our society as too individualistic or “go it alone”) and their vision of a wise redistribution of money, power, authority, rights, etc. often requires the correct person or group to be in charge. Having the Left’s preferred people in charge is, in fact, the preeminent value on the Left, and any other “rule” can be broken in its name – i.e., it’s okay to serve on corporate boards and make lots of money, as long as you donate to the party, etc.
In short, the rights of a female employee of San Diego Mayor Bob Filner don’t amount to that much in the minds of a lot of San Diego Democrats, compared to the need to keep Filner in charge so he can enact their preferred policies.