It's like the classic line in Sopranos
(or some such gangster type thing) where this guy asks his mate why he always gets hookers to suck him off, when he's got a nice attractive wife, to which he answers "she kisses our kids goodnight with that mouth!"
(
Mr. Tea 'ulmmm'/'mmneurgh', Fri 8 Apr 2005, 12:25,
archived)