While not a perfect equivalence, think of evolutionary stages like human stages of life. The most basic stage is usually a pokémon’s childhood, the middle stage is adolescence, and the final stage is adulthood. Now, granted, this isn’t a literal comparison. Pokémon in their final stages never see mating with basic-stage forms as the same thing as pedophilia, and breeders generally don’t, either. But in some cases, pokémon understand that there is some difference between their form and another form, even if that difference is vague and difficult for the pokémon themselves to define.
That having been said, it’s very likely that your luxio has been put off your luxray because there is a bit of a physical difference. Your luxray is about as mature as he will ever get, and thus, his body chemistry is slightly different. Perhaps his maturity is too, and neither you nor he can perceive that. Your luxio, meanwhile, does see a difference and may be experiencing uneasiness towards your luxray or even jealousy over his evolution.
At this point, it’s important to establish communication with your luxio to determine which it might be. Ask your luxray to wait in another room or as far from your luxio as possible, then sit down with your luxio and ask her what’s on her mind. If she looks agitated or restless, ask her if she’d like to evolve and gauge her reaction. If she seems interested in that question, form a plan to give her extra training and establish a consistent and daily routine until she evolves. Then reintroduce her to your luxray and see what happens.
If she looks a little more sullen, however, it’s very possible that she simply sees your luxray as a completely different pokémon. Do your best to reassure her that your luxray is still the same as always. If possible, after speaking with her, reintroduce her to your luxray gradually, as if introducing her to a new member of your team. Have your luxray give her a bit of space at first and have them work towards building a new relationship from the ground up. Starting basically from scratch allows her to approach the relationship while feeling less pressure to connect immediately with a pokémon she basically considers to be a stranger, which in turn means she’ll be far less on-edge around your luxray.
If all else fails, however, inform your luxray that she’s letting him down gently and perhaps get a breeding partner for them both. (It’s also possible that your luxio has already taken care of that part or that she has already laid an egg with either your luxray or another pokémon. I would observe her a bit before sitting down with her as well.)
Good luck, anonymous.