pf:

On no discussion in NT of polygamy as a sin: Do not get caught in the word-concept fallacy. To define marriage as between a man and a woman (as Jesus does) and to speak against adultery is to call polygamy a sin. One does not have to use the word to have the concept. In addition, I Cor 7 does speak of a person having his/her own spouse in the singular (vv 2, 39).

Also to change a law is not necessarily to change one's mind and be vacillating. Some of these practices were tied to cultural practices and associations tied to idolatry, which is why Israel was not to perform them at one point. As these practices lost that significance, they lost the need for performing them. The adminstrative differences Jesus introduced were part of a program associated with the new era Jesus brought (why he spoke about new wineskins). The situation with the war and taking of the land is unique in the OT and also is tied with dealing with idolatry and child sacrifice tied to such worship at the time.

Again none of this touches directly on the issue of gay marriage. 

dlb