Don't answer this question set until after you've read Chapter Thirty. Spoilers ahead!
Also: don't answer it if you're read past Chapter Thirty and have more information about what happens.
Rye claims that he lied to Frankie about his marital status because he loved Frankie, he believed that she loved him (or could love him), and he knew that if she knew he was married, she wouldn't enter into a relationship with him.
If you knew that the only way you could have a relationship with a person you believe might be the "love of your life," would lying to them be worse than not having a chance for a relationship with them at all? Simply put, is it okay to lie for a chance to find the love of your life?
Optional extra: see Sliding Doors (1998) for a film exploration of this question (not a requirement for answering the questions above).