Kayneth himself tells us that even the lowest class of Magi (anyone with magic circuits and some training in using them at the simplest levels) are functionally immune to Lancer's enchanting beauty mark. All they have to do is cycle their prana and it will wipe out the effects of the curse. In the anime in particular you actually see Iri start to come under the effect of the curse, only to immediately break it as soon as she suspects something unnatural is afoot. This happens in the novel, as well.
Sola-Ui herself is from a respected magus family, and while not the heir to the crest she was not ignorant of such basic techniques (by Kayneth's remarks). Her marriage to Kayneth is specifically an attempt to combine two high quality magi in hopes they will have similarly gifted children, which is itself an attempt to counter the degradation in the quality of magecraft and magi in the modern age.
However, the curse breaking only triggers if the affected person actually does the prana cycling, and so anyone unwilling (or unable) to do so will be subjected to the effects of the curse normally. I don't recall the novel ever giving us any of Sola-Ui's motivations direct from herself, but it is heavily implied that she intentionally allowed herself to be subjected to the curse. In the anime there is a sequence where she looks upon Lancer, we get a close-up of his beauty mark (the embodiment of the curse), and then a shot of an obviously entranced Sola-Ui, and a reaction shot of Kayneth suggesting he has noticed something happening. Kayneth also notices this in the novel, specifically telling the reader (through his internal monologue) that any magus should be immune as long as they try to resist, and then wrestling with the implications of whether Sola-Ui just intentionally let herself come under its influence.
So the answer to your question depends heavily on what one interprets that choice to mean.
Given that the novel was written by Urobuchi, who is well-known as a writer of tragic or at best bitter-sweet stories, would suggest that the correct interpretation of Sola-Ui's motivations here are lust and escape from an arranged future she does not desire. Romantic relationships in Fate/Zero, as with Urobuchi's other works, are inherently all flawed and tragic. As you said, she was not particularly interested in Kayneth, and certainly not in love with him (and even Kayneth himself acknowledges this, but hopes that with time she will come to love him in earnest). Lancer, on the other hand, was very attractive, mysterious, and possessing a romantic legend.
