There's an 'antipetobsession' mod in the Pets testing area, found here. I think it actually curbs the constant pet-related wants, but I'm not 100% sure about this.
This worked great for me! I haven't tested it on sims at uni or in households where the pet has been sold, but on the sims I have played pet wants now only come up occasionally.
Thanks so much
notveryawesome!