Maybe he did love Solo regardless, but it wasn't always a romantic love, not like he felt with Gaby.
"The difference is that I cannot imagine my life without you," Illya told her. "The very idea of it makes me ill. If we were not together, I would be in despair. I love you with all of me, Gaby. All I want is to be able to make you happy."
He hoped that he was making sense, that he was convincing her of his love without sounding like he was pleading for her.