Y'all underestimating the proximity effect and how easy it is to cheat on a film set
You are around these people for 12-16 hours a day, you ain't seeing your significant other like that unless they come there and they would need to contact you to even know where y'all filming at that day, actors go in each others trailers all the time to run lines and just shoot the shyt inbetween scenes, only takes a couple of minutes to fukk and get back on task.
Not to mention they are professional ACTORS, meaning they are in the top .01% of people who can fake you out and there ain't shyt you can do about it. I don't care how faithful you are, if a 10/10 woman with money is rehearsing kissing scenes with you periodically, eventually you're both gonna say fukk it

especially if y'all did multiple projects
I've seen assistant directors with their wedding band on taking hair and makeup women to hotel rooms and smashing with no fukks given, being married doesn't mean shyt on film sets brehs, trust me.