Actually, by Catholic standards they were free to marry, since Joy’s first husband had himself been married before.
Actually, they had more trouble with the C of E than they would have had with the Catholic Church. The C of E had, and still has for the most part, a pretty strict policy against church weddings for divorced people. This is a point on which Catholics have a completely wrong view of the C of E. (Henry VIII got an annulment, and he was an exception to all the rules anyway.)
A complication for Lewis and Gresham is that they had a civil marriage some time before they got a religious marriage. Lewis claimed that the former was purely to let her stay in the country, which would indicate that it was unconsummated. Some have suggested that maybe they did in fact have a sexual relationship at this point (after the civil marriage but before the religious one), and I wouldn’t be surprised if some scandal-mongers have claimed that they had one even earlier. On the other hand, some hard-core Lewis fans have claimed that they never had sex at all (which IMHO is ridiculous).
A much more reasonable speculation is that Lewis may have had a sexual relationship with Mrs. Moore, also a divorced woman (actually I’m not sure if she was legally divorced) with whom he lived for years in fulfillment of a promise made to her son who had been killed in WWI. Lewis generally referred to her as his “mother,” but I’ve heard claims (at the New York C. S. Lewis Society, a pretty respectable place to get info about Lewis) that Maureen (Mrs. Moore’s daughter) later reported having seen them behaving in ways that pointed toward a different kind of relationship.
I hate even to post any of this. Lewis was such a private person, and I’d like to be able to respect that. But given that there are a lot of wild claims out there, I wanted to state the facts as I know them.
P.S. I don’t think anyone reasonably disputes that if there was a sexual relationship with Mrs. Moore, it would have ended at or before the time of Lewis’s conversion to Christianity.