Within the ten commandments, thou shall not commit adultery (here..
http://en.wikipedia.org/wiki/Thou_sh...ommit_adultery), explains "Christian Churches view adultery as sexual relations in which at least one participant is married to someone else. According to the Genesis narrative, marriage is a union established by God:."
So I would think based on the definition or commandment, swinging is a sin in the eyes of the christian church as well as seeing married women/prostitutes. Or in hindsight women/prostitutes seeing married men.
As far as homosexuality, there are a number of verses explaining it to be a sinful act within the bible.
http://www.openbible.info/topics/homosexuality
In answering the OP question, if neither party is married, there is no sin from what I can gather. In turn, based on these findings, there is a highway to hell and we're on it.