Jerry had an affair, Lucy didn't
Read a few comments saying that it is indeterminate if Jerry had an affair when he told Lucy he was in Florida. The awful truth is he did have an affair. A few pieces of evidence:
1. At the end of the movie, he said to Lucy--I paraphrase here--that he had changed during their marriage, but things can go back to be the same if she allowed him to.
2. He's a chronic liar. He lied to Barbara Vance when he could have told her the truth.
3. Because he just had an affair, when he returned home and didn't find Lucy, he suspected her of an affair too.
4. If he didn't have an affair, he would probably tell the guy in the gym room where he had been or what he had done. If he didn't have an affair, we as audience would know what he had done. But we weren't told because he did have an affair but at the same time, we had to keep a sympathetic view on Jerry. Cary Grant probably didn't want to ruin his image by being an adulterer.
Lucy, on the other hand, didn't because:
1. She was squemish even about kissing Dan Leeson because she didn't love him, showing she couldn't be intimate with someone she didn't love and she loved only Jerry.
2. (This point is said in jest) The singing teacher is probably gay. When Lucy said something to the effect of, "Jerry barged in because he cared too much for me."
The teacher said wistfully, "And he cared too little for me."
He's French(?) and teaches singing! How can he not be gay?