Coincidentally enough, this sort of goes hand in hand with this open marriage idea I was about to post:
This married couple has complete knowledge of them sleeping with other people. It's not uncommon for one of them to stay out the night with a sex partner or two, for them to go out on a date and both leave with another person or to bring them home. Another thing they did was group sex - from twosomes with the other spouse watching, threesomes - both MMF & FFM, foursomes and etc.
The wife would watch her husband get fucked by another man and while the husband is fucking her, the man is fucking him as well. If there is another male, the wife and husband would alternate sucking his cock.