Necessity rhymes with:
analyticity, animosity, atrocity, audacity, biodiversity, capacity, complexity, complicity, curiosity, density, diversity, domesticity, eccentricity, elasticity, electricity, falsity, felicity, ferocity, generosity, incapacity, intensity, laxity, monstrosity, obesity, opacity, overcapacity, paucity, perplexity, perversity, publicity, religiosity, simplicity, specificity, toxicity, university, velocity, viscosity, voracity
Necessity sounds like:
nagata, nagged, nakata, naked, nast, nasta, nasty, naught, naughty, nauseate, nauseated, necessitate, necessitated, necked, necktie, negate, negated, negotiate, negotiated, nesseth, nest, nesta, neste, nestea, nested, neustadt, newest, newgate, newgateway, newquist, newscast, newsday, next, nicest, nicety, nicked, niezgoda, night, nikita, niquette, nishida, nist, nixed, njt, noisiest, nonjet, nonunionized, nosed, nuanced, nugett, nugget, nunziata, nunziato, nyquist