As you hinted at, Buchanan and King not only were rumored to be Friends of Dorothy, but to be a couple. I’m not sure how much of that merely can be attributed to the virulent politics of the 19th Century.
Senator Pete Domeneci was married throughout his Senate years and never dated any fellow members, but he did manage to knock up Senator Paul Laxalt’s daughter.
I missed that hilarious news. Pete, you dog.
The news story I found linked to one about TN Congressman Steve Cohen having a love child of his own.
http://abcnews.go.com/blogs/politics/2013/02/rep-cohens-tweets-reveal-secret-daughter/