by oldtwit
for 750 words it was good, obviously limited by word count so it wasn't as "smooth" to read. No surprise there.
Expand that story and it could a good swap-partner-to-lovers romance, especially if you "demonize" Su and Ian a little (such as they were caught meeting outside the rules or Ian screwed around and brought a mild STD into the group) and have the MC be introspective and realize what his feelings for Ann. "I have never been in love before and didn't know what love felt like so I thought what I was feeling for Su was love... then I met you."
It would also be a warning to the downside of a swinging lifestyle, there's always the chance that your partner develops feelings for another.
Too many run-on sentences, where you forego periods for commas, because you don't know how to punctuate, but it's really simple, people stop speaking at the end of a sentence, but when they pause it's where you put a comma, that's the simple answer, English when written is more complex, because when we read we need something to take over for inflection, eye contact, timing and those other non-verbal elements which make up about 905 of person-to-person communication, but there are all kinds of writing tools on the Internet, my Grammarly says this paragraph contains 45 errors, which mirrors your writing.