Maybe the quandary is missing the point entirely. There are loads of reasons for soft swap beyond the physical, emotional and watching what's going on instead of participating. A friend contracted HPV her first time at a house party; and how about fluid bonding with strangers, STD's - ED - stage freight - feminine dryness - and a couple wanting to take baby steps in dangerous waters? We're a full swap couple and we think they're all great reasons in our book for soft swinging.
Our best lovers started soft and became hard stoppers after they gained their footing, confidence, and understanding of a lifestyle marriage. There's also been a time or two with good friends when intercourse wasn't possible (periods, UTI) and those were the most memorable times. While it didn't stop the intimacy or passion of seeing FWB, it did curtail the intercourse on one side of the swap. From those instances both of our marriages gained. We learned about participating in your partners ecstasy by watching, treating your lover like your wife, and being satisfied with what you have already. They were all great lessons we didn't know we needed to learn. We came away with hot memories and ever since the sex with these friends has been off the charts delicious.
As for parallel play where you fuck your wife while I fuck mine - we're satisfied if we can get our hands on the other couple while we're all in the mix, but we don't find it erotic to have close-by sex and no physical contact (that's just is.) To start, we always wanna spread the Mrs, play and suck on her clit, enjoy 4-4 O's each, then one of us to glide her husband-like cock into her hole. That's soft fun for us.
Living all summer on a clothing optional resort we are immersed in all kinds of soft swing hotness; nudity, flashing, masturbation, erotic massages, g/g, oral, and hand jobs in the shower... they're great ways we say "hello," to familiar friends. We see fast-track lovers, who can't wait to plunge their cock into something new, go over the cliff when they realize their lover's gone wild with somebody new trying something "new" like kissing, stroking, caressing and kissing. It's the old "I wanna bang your girl but don't making passionate love to my wife."
We love it all and identify that each couple sets their own pace and for their own reasons. Investigating the couple and their reason(s) for soft swinging might make a whole heck of a lot of difference in the end, and you might unlock a few hidden secrets about yourself and your partner like we did.