According to this report (and others on the Internet), Hillary may prefer sex with women to sex with men:
Does that imply that Bill is Hillary's "merkin"?
A merkin in the gay and lesbian set in Hollywood refers to a "usually" straight male escorting a gay woman to public events so as to attempt to camouflage the lesbian's secret sapphic lifestyle.
Source - http://www.urbandictionary.com/define.php?term=merkin&page=2
Were that to be true, it would explain why Hillary seems to be so amused by Bill's "infidelities", which are thereby revealed to be just part of their charade.