Just 2 different spectrums.
The GoP tends to be more conservative/libertarian on social issues.
Some support SSM and MJ legalization, but many oppose it due to their belief that it would negatively impact society or it isn't something that is needed/will be abused.
On the other hand, Democrats tend to be more liberal on social issues leaving society to do what it wants unregulated by government.
Where this changes is when it comes to money, economics and other aspects of life. Dems typically want more government control, more taxes, more social programs, more welfare and more regulation of business/the economy where as Republicans tend to oppose or be less imposing in these areas.
I'm all for states legalizing SSM on their own, I'm not for legalizing MJ. Each party has their best view for society, with one being more restrictive on some personal actions (abortion, homosexual marriage, illegal drug use, etc) while the other being more restrictive on some economic actions (higher taxes, more government control, more social programs, more regulation of business, etc). They each have their reasons as to why they are right. The reason Democrats tend to back SSM and MJ in my opinion, is to appeal to their constituents and go along with issues that fit in line with how they want to mold society. Republicans do the same.