As you said, with men becoming more effeminate, it's not so surprising this is a rising trend. I can't imagine most masculine men being willing to participate in a MMF threesome (what they used to call a "devil's threesome" here, lol), or allow another male on his territory banging his wife.
Also, there's the increase of the "hookup" culture, and the continued deterioration of traditional values, so again it's not surprising to see other people being brought into the marriage bed.
Personally, I've never been into the group sex thing, but YMMV.