Yes, someone who is married can tour the theatre (and other rabbitholes too; City Hall and the science lab, off the top of my head) and make out with someone who isn't their spouse, and it's a pretty universal thing for touring rabbitholes. The funny thing is that the game doesn't count it as cheating; they're not made into romantic interests by making out in the theatre and their spouse will not accuse them of cheating for doing it. You can even have the same sim woohoo/Try for Baby with someone else while touring the theatre (has to be someone you made out with in the theatre first, though) and still not technically cheat on their significant other.
