I was told no unprotected sex before FET. So use a condom. After FET it was then not advised until test day and if positive then not until after the scan.
We were told not to because they didn't want to risk you getting a uti which could possibly negatively impact the outcome
I’m certain before FET is fine just after I think they advise not to as they don’t want any contractions/ spasms. I think before is good get the blood flow pumping in the area.