You never have to say "I love you." We are only called to love, not to call attention to it.
In this case, I would actually advise against saying "I love you." Of course you care about your parents and their new pseudo-spouses, and you show your love in all sorts of ways. Perhaps, since your parents' new "significant others" are acting as your new parents, they expect the sort of verbal statements of filial devotion (like, "I love you") that most parents expect.
But their new "significant others" are not your parents. They are, indeed, people who are living in sin with your parents, and -- not to be rude to anyone in your family, as I'm sure they are all very nice people -- they have exposed themselves and your parents to a very serious risk of permanently separating themselves from God's love through a kind of marital selfishness. They do not deserve, and can never earn, the particular kind of love you owe to your parents. If you choose to say that you love them, that's perfectly okay, and, because you are a good Christian, it is no doubt true. But you are under no moral obligation whatsoever to say anything of the kind, especially if it would encourage them to think that what they're doing is a-okay with you or with God.
I really hope that doesn't sound too harsh. I hate to be the guy throwing hellfire around the thread... but your obligations to your father's girlfriend and mother's new "husband" begin and end with the Christian call to love thy neighbor as thyself. You don't need to feel guilty for failing to give them the special devotion typically reserved for parents. I hope you find this answer comforting, rather than offensive -- but I'm aware there's a risk of the latter!