It varies state-by-state of course. In Virginia, there are multiple "strike" statutes. The normal three strikes one covers violent felonies (with a mandatory life sentence). There's a two strikes one for lower-level sex felonies (with the punishment being the max sentence, but not life), and another one for major sex felonies (with the punishment being life).
All of them are for felonies.
Some three strikes laws have been far less discerning. California used to have one of the stupidest ones in the country. After two violent felonies, your third strike could be something stupid like shoplifting. They have since fixed it but for a while there were people who were sent away for life for stealing DVDs.
EDIT: Incidentally, felony incest is not a strike in Virginia unless it's with your child or grandchild, so Chris doesn't get a strike. (Oddly, felony oral/anal with your mother IS a strike, but Chris isn't charged with that.)