In Illinois, between 1960-68, what type of criminal act was adultery? 
What were the possible punishments to a defendant if prosecuted?

Hello.

During the 1960s, adultery was punishable as a misdemeanor in
Illinois. Misdemeanors, then and now, are punishable by imprisonment
of up to one year.

According to the Illinois Criminal Code of 1961, still in effect,
adultery is a "Class A" misdemeanor.
(720 ILCS 5/11-7) Sec. 11-7. Adultery.) (a) Any person who has sexual
intercourse with another not his spouse commits adultery, if the
behavior is open and notorious, and (1) The person is married and the
other person involved in such intercourse is not his spouse; or (2)
The person is not married and knows that the other person involved in
such intercourse is married. A person shall be exempt from prosecution
under this Section if his liability is based solely on evidence he has
given in order to comply with the requirements of Section 4-1.7 of
"The Illinois Public Aid Code", approved April 11, 1967, as amended.
(b) Sentence. Adultery is a Class A misdemeanor.
http://www.llcc.cc.il.us/gtruitt/SCJ%20100%20Fall%202002%20Start%20Page/ilcs%20sex%20offenses.htm


According to Illinois' Unified Code of Corrections, Class A
misdemeanors are punishable by imprisonment of "any term less than one
year."
See the code:

730 ILCS 5/
(730 ILCS 5/5-8-3)
    Sec. 5-8-3.  Sentence  of  Imprisonment  for  Misdemeanor.)   (a) 
A
sentence  of  imprisonment  for a misdemeanor shall be for a
determinate
term according to the following limitations:
    (1)  for a Class A misdemeanor, for any term less than one year;
http://www.legis.state.il.us/ilcs/ch730/ch730act5articles/ch730act5sub28.htm


Even though the adultery law has been on the books for years, it has
rarely been enforced. Back in 1997, in Harvey, Illinois, a married
woman and a man with whom she was allegedly having an affair were
charged with adultery under the law. The local district attorney
declined to prosecute the case, however. For more information, see a
pair of articles in the archives of the Chicago Tribune:

ALLEGED TRYST REVIVES RARE ADULTERY LAW
Chicago Tribune; Chicago, Ill.; Jul 11, 1997; Joseph Tybor ;Jerry
Thornton, Tribune Staff Writers
http://pqasb.pqarchiver.com/chicagotribune/abstract/12940856.html?did=12940856&FMT=ABS&FMTS=FT&desc=ALLEGED+TRYST+REVIVES+RARE+ADULTERY+LAW

DEVINE WON'T PROSECUTE ADULTERY CASE ENFORCING LAW SEEN AS
PROBLEMATIC, COSTLY
Chicago Tribune; Chicago, Ill.; Jul 12, 1997; Joseph Tybor ;Jerry
Thornton, Tribune Staff Writers.
http://pqasb.pqarchiver.com/chicagotribune/abstract/12958402.html?did=12958402&FMT=ABS&FMTS=FT&desc=DEVINE+WON%27T+PROSECUTE+ADULTERY+CASE++ENFORCING+LAW+SEEN+AS+PROBLEMATIC,+COSTLY
Search strategy: illinois, "commits adultery," misdemeanor

I hope this helps.

Work was clear, concise, to the point, and just alittle more then required,nice.