What is it that makes people (try) to be monogamous?

Tonight I went to class and we were discussing the psychology of human pairing and relationships. Now I accept the fact that pairing is, from an evolutionary point of view, a necessity. However, the Evolutionary Psych point of view leaves itself open to criticisms, such as in the case of homosexuality, infertility, birth defects (specifically of the reproductive organs and the brain), and most importantly, of why there is such a desire to become paired. I've read many studies and seen many statistics on depression, anxiety, and suicide, and one of the leading causes seem to be this phenomena of human bonding; whether it be an individual in a relationship, or lack thereof. This seems counter-intuitive to evolution as this behavior, while beneficial to procreation, may also be detrimental to the individual(s) involved. Likewise, it still doesn't explain why there is (pardon the Freudian term) such a drive for humans to WANT to have relationships. Keep in mind, that procreation can and does happen outside of relationships (i.e. single parent families, etc.) As a I mentioned before homosexuality really complicates things even more. Individuals in relationships, especially long-term ones, consistently rate their happiness higher. I don't see a contradiction there: relationships increase the odds and survivability of offspring, so we're designed to want to be in them. Homosexuality may not be adaptive, or it may. Looking into kin selection, which makes sense in this context. Or it could be something else entirely. Homosexuality exists throughout the animal kingdom, so it may serve the same purpose in humans, or not; human sexuality is complicated.