Here are some standard replies (some worse than others): (1) to reduce your chances of contracting an STD. (2) because you barely have enough time and energy and money for one relationship, let alone two. (3) because God said so, and you shouldn't tamper with Him. (4) because your mama said so, and you shouldn't tamper with Her. (5) because your significant other (SO) said "no way, jose" to your group sex proposal. (6) because you promised to be monogamous and it is wrong to break promises, ceteris paribus. can you think of a situation that would permit the breaking? maybe: your SO already broke it. (7) because it is the best arrangement for the rearing of children. (8) because from 4 to 8 years of monogamy is built into us genetically and we shouldn't tamper with Nature. (9) because monogamy (even if not followed by 50-80% of those who claim to follow it) supports the economy better than does polygamy. (Part of my problem is that I'm not sure whether your "why" asks for causes or reasons.)