At least in the show, the situation was not quite as the question describes it. Unity actually met Desire in the Dreaming (or possibly some spinoff of it like the one Gault made), and they started a relationship.
Unity: I fell asleep, and I didn't wake up. Until eight months ago. But in my dreams, I had the most glorious life. I took over my father's business. And I met a man with golden eyes. And we had a baby.
The Sandman, "The Doll's House" (S01E07)
Despite Unity describing it as "not real," the Dreaming is as real as the waking world, and Unity was completely aware of her actions there (due to being the intended Vortex?), so it is natural that she remembered it.
With that in mind, Unity views their relationship favorably because from her perspective it was entirely consensual. She met Desire, they fell in love (from her perspective), and they had a child. Only it happened in the Dreaming, not the waking world.
Unity: I was meant to have died a long time ago, Rose. But if I had, I would never have met my golden-eyed man and we would never have had our beautiful baby girl, and you would not have been born.
The Sandman, "Lost Hearts" (S01E10)
Now from an out-of-universe perspective, there are certainly reasons to at least be somewhat skeptical that the relationship was as consensual as all that. After all, Desire did not tell Unity that they were only having a relationship with her in order to conceive a Vortex that would be related to Dream. Furthermore, it is probably safe to say that Desire has a supernatural influence over people's affections.
Still, without Unity knowing either of these facts, nor indeed the true identity of Desire, all she knows is that she met a beautiful person in the Dreaming and had a child with them, so it is perhaps not so surprising that she looks upon their relationship favorably.