Mc and his wife swings with the parents of him

[Lumaloam]

The story occurs in a camp

 

[Hypoxia]

Make it a church camp run by the Church of the Naked Jesus or a congregation of Extra-Fundamental Baptists (same thing). Lots of skyclad re-baptisms aka cleanups before and after the mini-orgies. Swinging is traditional, obeying the Savior's admonition for followers to share everything, especially each other.

 

[AvroAnsonXIX]

Extra-Fun. Baptists ...

Naked rebaptisms (nudity being symbolic of our rejection of the temporal) in the church swimming pool every Sunday. Right before the partaking of and gifting of the Holy Spirit (as contained in our essential essences) to one's "brothers and sisters" in the Lord (it's okay if they aren't blood so long as you "adopt" them first).

 

[silversword]

The story occurs in a camp

I would think most wives consider swinging a bit of a bold step in a relationship, finding the right partners that they find attractive but not likely to cause long term issues... the in-laws would seem the worst choice ever.

Unless, say, they had invented a time machine and decide to go back to when his parents were of a similar age. The question is he ready to find out what his parents were really like

 

[Hypoxia]

...the in-laws would seem the worst choice ever.

Unless the couple first met at a family-friendly swing club. The daughter was the MC's second choice for a long-term linkup but her mom was already taken.

Unless, say, they had invented a time machine and decide to go back to when his parents were of a similar age. The question is he ready to find out what his parents were really like

My grandparents were rum-runners. My partner's grandma was a swinging party girl. Nonstandard lifestyles were not invented in a recent generation.

To see what your folks were like, dig into their stash of home videotapes.