My guess is after visiting his sister who recently had a baby it made him realize what sex is all about....and that made him not want to do it at the time. But a threesome also made him realize what sex can be about too and brought back his interest in it.
Having a baby can really mess up a guy for a while...that's why I notice some leave their wives soon afterwards...maybe because it is difficult to see her as more than a mother. Doing really freaky things can help a man become interested in sex again...especially with a wife who recently has had a baby.