There are several issues with this claim.
Falling in love with a fantasy may hold true for long-term infidelity, where feelings have been allowed to grow. Yet, this does not explain instances of "unplanned infidelity", where the cheating party has not met the person beforehand.
Research shows that these idealised projections have a lifespan. While lust can obscure the realities of a person, it cannot do so forever. In affairs, where unions can last years, lust does not explain how these develop and sustain.